diff --git a/backend/app/main.py b/backend/app/main.py
index 51e8eef..9744f88 100644
--- a/backend/app/main.py
+++ b/backend/app/main.py
@@ -371,7 +371,7 @@ async def root():
 
 
 # Include V2 API routers
-from app.routers import auth, tasks, admin
+from app.routers import auth, tasks, admin, translate
 from fastapi import UploadFile, File, Depends, HTTPException, status
 from sqlalchemy.orm import Session
 import hashlib
@@ -385,6 +385,7 @@ from app.services.task_service import task_service
 app.include_router(auth.router)
 app.include_router(tasks.router)
 app.include_router(admin.router)
+app.include_router(translate.router)
 
 
 # File upload endpoint
diff --git a/backend/app/routers/__init__.py b/backend/app/routers/__init__.py
index de393ce..5a0bf08 100644
--- a/backend/app/routers/__init__.py
+++ b/backend/app/routers/__init__.py
@@ -2,6 +2,6 @@
 Tool_OCR - API Routers (V2)
 """
 
-from app.routers import auth, tasks, admin
+from app.routers import auth, tasks, admin, translate
 
-__all__ = ["auth", "tasks", "admin"]
+__all__ = ["auth", "tasks", "admin", "translate"]
diff --git a/backend/app/routers/translate.py b/backend/app/routers/translate.py
new file mode 100644
index 0000000..d42681f
--- /dev/null
+++ b/backend/app/routers/translate.py
@@ -0,0 +1,503 @@
+"""
+Tool_OCR - Translation Router
+Handles document translation operations via DIFY AI API
+"""
+
+import logging
+import json
+from datetime import datetime
+from pathlib import Path
+from typing import Optional
+
+from fastapi import APIRouter, Depends, HTTPException, status, Query, BackgroundTasks
+from fastapi.responses import FileResponse, JSONResponse
+from sqlalchemy.orm import Session
+
+from app.core.deps import get_db, get_current_user
+from app.core.config import settings
+from app.models.user import User
+from app.models.task import Task, TaskStatus
+from app.schemas.translation import (
+    TranslationRequest,
+    TranslationStartResponse,
+    TranslationStatusResponse,
+    TranslationStatusEnum,
+    TranslationProgress,
+    TranslationListResponse,
+    TranslationListItem,
+    TranslationStatistics,
+)
+from app.services.task_service import task_service
+from app.services.dify_client import LANGUAGE_NAMES
+
+logger = logging.getLogger(__name__)
+
+router = APIRouter(prefix="/api/v2/translate", tags=["Translation"])
+
+
+def run_translation_task(
+    task_id: str,
+    task_db_id: int,
+    target_lang: str,
+    source_lang: str = "auto"
+):
+    """
+    Background task to run document translation.
+
+    Args:
+        task_id: Task UUID string
+        task_db_id: Task database ID (for verification)
+        target_lang: Target language code
+        source_lang: Source language code ('auto' for detection)
+    """
+    from app.core.database import SessionLocal
+    from app.services.translation_service import get_translation_service
+    from app.schemas.translation import TranslationJobState, TranslationProgress
+
+    db = SessionLocal()
+    translation_service = get_translation_service()
+
+    try:
+        logger.info(f"Starting translation for task {task_id} -> {target_lang}")
+
+        # Get task to find result JSON path
+        task = db.query(Task).filter(Task.task_id == task_id).first()
+        if not task:
+            logger.error(f"Task {task_id} not found")
+            return
+
+        if not task.result_json_path:
+            logger.error(f"Task {task_id} has no result JSON")
+            translation_service.set_job_state(task_id, TranslationJobState(
+                task_id=task_id,
+                target_lang=target_lang,
+                source_lang=source_lang,
+                status=TranslationStatusEnum.FAILED,
+                progress=TranslationProgress(),
+                error_message="No OCR result found",
+                started_at=datetime.utcnow()
+            ))
+            return
+
+        result_json_path = Path(task.result_json_path)
+        if not result_json_path.exists():
+            logger.error(f"Result JSON not found: {result_json_path}")
+            translation_service.set_job_state(task_id, TranslationJobState(
+                task_id=task_id,
+                target_lang=target_lang,
+                source_lang=source_lang,
+                status=TranslationStatusEnum.FAILED,
+                progress=TranslationProgress(),
+                error_message="Result file not found",
+                started_at=datetime.utcnow()
+            ))
+            return
+
+        # Update state to translating
+        translation_service.set_job_state(task_id, TranslationJobState(
+            task_id=task_id,
+            target_lang=target_lang,
+            source_lang=source_lang,
+            status=TranslationStatusEnum.TRANSLATING,
+            progress=TranslationProgress(),
+            started_at=datetime.utcnow()
+        ))
+
+        # Progress callback
+        def progress_callback(progress: TranslationProgress):
+            current_state = translation_service.get_job_state(task_id)
+            if current_state:
+                current_state.status = TranslationStatusEnum.TRANSLATING
+                current_state.progress = progress
+                translation_service.set_job_state(task_id, current_state)
+
+        # Run translation
+        success, output_path, error_message = translation_service.translate_document(
+            task_id=task_id,
+            result_json_path=result_json_path,
+            target_lang=target_lang,
+            source_lang=source_lang,
+            progress_callback=progress_callback
+        )
+
+        if success:
+            translation_service.set_job_state(task_id, TranslationJobState(
+                task_id=task_id,
+                target_lang=target_lang,
+                source_lang=source_lang,
+                status=TranslationStatusEnum.COMPLETED,
+                progress=TranslationProgress(percentage=100.0),
+                started_at=datetime.utcnow(),
+                completed_at=datetime.utcnow(),
+                result_file_path=str(output_path) if output_path else None
+            ))
+            logger.info(f"Translation completed for task {task_id}")
+        else:
+            translation_service.set_job_state(task_id, TranslationJobState(
+                task_id=task_id,
+                target_lang=target_lang,
+                source_lang=source_lang,
+                status=TranslationStatusEnum.FAILED,
+                progress=TranslationProgress(),
+                error_message=error_message,
+                started_at=datetime.utcnow()
+            ))
+            logger.error(f"Translation failed for task {task_id}: {error_message}")
+
+    except Exception as e:
+        logger.exception(f"Translation failed for task {task_id}")
+        translation_service.set_job_state(task_id, TranslationJobState(
+            task_id=task_id,
+            target_lang=target_lang,
+            source_lang=source_lang,
+            status=TranslationStatusEnum.FAILED,
+            progress=TranslationProgress(),
+            error_message=str(e),
+            started_at=datetime.utcnow()
+        ))
+
+    finally:
+        db.close()
+
+
+@router.post("/{task_id}", response_model=TranslationStartResponse, status_code=status.HTTP_202_ACCEPTED)
+async def start_translation(
+    task_id: str,
+    request: TranslationRequest,
+    background_tasks: BackgroundTasks,
+    db: Session = Depends(get_db),
+    current_user: User = Depends(get_current_user)
+):
+    """
+    Start a document translation job.
+
+    - **task_id**: Task UUID of a completed OCR task
+    - **target_lang**: Target language code (e.g., 'en', 'ja', 'zh-TW')
+    - **source_lang**: Source language code ('auto' for automatic detection)
+
+    Returns 202 Accepted with job information. Use /status endpoint to track progress.
+    """
+    from app.services.translation_service import get_translation_service
+    from app.schemas.translation import TranslationJobState
+
+    # Get task
+    task = task_service.get_task_by_id(
+        db=db,
+        task_id=task_id,
+        user_id=current_user.id
+    )
+
+    if not task:
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND,
+            detail="Task not found"
+        )
+
+    # Check task is completed
+    if task.status != TaskStatus.COMPLETED:
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail=f"Cannot translate task in '{task.status.value}' status. Task must be completed."
+        )
+
+    # Check result JSON exists
+    if not task.result_json_path or not Path(task.result_json_path).exists():
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail="OCR result not found. Please process the document first."
+        )
+
+    # Validate target language
+    target_lang = request.target_lang
+    if target_lang not in LANGUAGE_NAMES:
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail=f"Unsupported target language: {target_lang}. Supported: {', '.join(LANGUAGE_NAMES.keys())}"
+        )
+
+    # Check if translation already exists
+    result_dir = Path(task.result_json_path).parent
+    existing_translation = result_dir / f"{Path(task.result_json_path).stem.replace('_result', '')}_translated_{target_lang}.json"
+    if existing_translation.exists():
+        logger.info(f"Translation already exists: {existing_translation}")
+        # Return as completed
+        return TranslationStartResponse(
+            task_id=task_id,
+            status=TranslationStatusEnum.COMPLETED,
+            target_lang=target_lang,
+            message="Translation already exists"
+        )
+
+    # Check if translation is already in progress
+    translation_service = get_translation_service()
+    current_job = translation_service.get_job_state(task_id)
+    if current_job and current_job.status in [TranslationStatusEnum.PENDING, TranslationStatusEnum.TRANSLATING]:
+        return TranslationStartResponse(
+            task_id=task_id,
+            status=current_job.status,
+            target_lang=current_job.target_lang,
+            message="Translation already in progress"
+        )
+
+    # Initialize job state
+    translation_service.set_job_state(task_id, TranslationJobState(
+        task_id=task_id,
+        target_lang=target_lang,
+        source_lang=request.source_lang,
+        status=TranslationStatusEnum.PENDING,
+        progress=TranslationProgress(),
+        started_at=datetime.utcnow()
+    ))
+
+    # Start background translation task
+    background_tasks.add_task(
+        run_translation_task,
+        task_id=task_id,
+        task_db_id=task.id,
+        target_lang=target_lang,
+        source_lang=request.source_lang
+    )
+
+    logger.info(f"Started translation job for task {task_id}, target_lang={target_lang}")
+
+    return TranslationStartResponse(
+        task_id=task_id,
+        status=TranslationStatusEnum.PENDING,
+        target_lang=target_lang,
+        message="Translation job started"
+    )
+
+
+@router.get("/{task_id}/status", response_model=TranslationStatusResponse)
+async def get_translation_status(
+    task_id: str,
+    db: Session = Depends(get_db),
+    current_user: User = Depends(get_current_user)
+):
+    """
+    Get the status of a translation job.
+
+    - **task_id**: Task UUID
+
+    Returns current translation status with progress information.
+    """
+    from app.services.translation_service import get_translation_service
+
+    # Verify task ownership
+    task = task_service.get_task_by_id(
+        db=db,
+        task_id=task_id,
+        user_id=current_user.id
+    )
+
+    if not task:
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND,
+            detail="Task not found"
+        )
+
+    # Get job state
+    translation_service = get_translation_service()
+    job_state = translation_service.get_job_state(task_id)
+
+    if not job_state:
+        # No active job - check if any completed translations exist
+        if task.result_json_path:
+            result_dir = Path(task.result_json_path).parent
+            translated_files = list(result_dir.glob("*_translated_*.json"))
+            if translated_files:
+                # Return completed status for the most recent translation
+                latest_file = max(translated_files, key=lambda f: f.stat().st_mtime)
+                # Extract language from filename
+                lang = latest_file.stem.split("_translated_")[-1]
+                return TranslationStatusResponse(
+                    task_id=task_id,
+                    status=TranslationStatusEnum.COMPLETED,
+                    target_lang=lang,
+                    progress=TranslationProgress(percentage=100.0)
+                )
+
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND,
+            detail="No translation job found for this task"
+        )
+
+    return TranslationStatusResponse(
+        task_id=task_id,
+        status=job_state.status,
+        target_lang=job_state.target_lang,
+        progress=job_state.progress,
+        error_message=job_state.error_message,
+        started_at=job_state.started_at,
+        completed_at=job_state.completed_at
+    )
+
+
+@router.get("/{task_id}/result")
+async def get_translation_result(
+    task_id: str,
+    lang: str = Query(..., description="Target language code"),
+    db: Session = Depends(get_db),
+    current_user: User = Depends(get_current_user)
+):
+    """
+    Get the translation result for a specific language.
+
+    - **task_id**: Task UUID
+    - **lang**: Target language code (e.g., 'en', 'ja')
+
+    Returns the translation JSON file.
+    """
+    # Verify task ownership
+    task = task_service.get_task_by_id(
+        db=db,
+        task_id=task_id,
+        user_id=current_user.id
+    )
+
+    if not task:
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND,
+            detail="Task not found"
+        )
+
+    if not task.result_json_path:
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND,
+            detail="OCR result not found"
+        )
+
+    # Find translation file
+    result_dir = Path(task.result_json_path).parent
+    base_name = Path(task.result_json_path).stem.replace('_result', '')
+    translation_file = result_dir / f"{base_name}_translated_{lang}.json"
+
+    if not translation_file.exists():
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND,
+            detail=f"Translation for language '{lang}' not found"
+        )
+
+    # Return as JSON response with proper content type
+    return FileResponse(
+        path=str(translation_file),
+        filename=translation_file.name,
+        media_type="application/json"
+    )
+
+
+@router.get("/{task_id}/translations", response_model=TranslationListResponse)
+async def list_translations(
+    task_id: str,
+    db: Session = Depends(get_db),
+    current_user: User = Depends(get_current_user)
+):
+    """
+    List all available translations for a task.
+
+    - **task_id**: Task UUID
+
+    Returns list of available translations with metadata.
+    """
+    # Verify task ownership
+    task = task_service.get_task_by_id(
+        db=db,
+        task_id=task_id,
+        user_id=current_user.id
+    )
+
+    if not task:
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND,
+            detail="Task not found"
+        )
+
+    translations = []
+
+    if task.result_json_path:
+        result_dir = Path(task.result_json_path).parent
+        translated_files = list(result_dir.glob("*_translated_*.json"))
+
+        for translation_file in translated_files:
+            try:
+                # Extract language from filename
+                lang = translation_file.stem.split("_translated_")[-1]
+
+                # Read translation metadata
+                with open(translation_file, 'r', encoding='utf-8') as f:
+                    data = json.load(f)
+
+                stats_data = data.get('statistics', {})
+
+                translations.append(TranslationListItem(
+                    target_lang=lang,
+                    translated_at=datetime.fromisoformat(data.get('translated_at', '').replace('Z', '+00:00')),
+                    provider=data.get('provider', 'dify'),
+                    statistics=TranslationStatistics(
+                        total_elements=stats_data.get('total_elements', 0),
+                        translated_elements=stats_data.get('translated_elements', 0),
+                        skipped_elements=stats_data.get('skipped_elements', 0),
+                        total_characters=stats_data.get('total_characters', 0),
+                        processing_time_seconds=stats_data.get('processing_time_seconds', 0.0),
+                        total_tokens=stats_data.get('total_tokens', 0)
+                    ),
+                    file_path=str(translation_file)
+                ))
+            except Exception as e:
+                logger.warning(f"Failed to read translation file {translation_file}: {e}")
+                continue
+
+    return TranslationListResponse(
+        task_id=task_id,
+        translations=translations
+    )
+
+
+@router.delete("/{task_id}/translations/{lang}", status_code=status.HTTP_204_NO_CONTENT)
+async def delete_translation(
+    task_id: str,
+    lang: str,
+    db: Session = Depends(get_db),
+    current_user: User = Depends(get_current_user)
+):
+    """
+    Delete a specific translation.
+
+    - **task_id**: Task UUID
+    - **lang**: Target language code to delete
+    """
+    # Verify task ownership
+    task = task_service.get_task_by_id(
+        db=db,
+        task_id=task_id,
+        user_id=current_user.id
+    )
+
+    if not task:
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND,
+            detail="Task not found"
+        )
+
+    if not task.result_json_path:
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND,
+            detail="OCR result not found"
+        )
+
+    # Find translation file
+    result_dir = Path(task.result_json_path).parent
+    base_name = Path(task.result_json_path).stem.replace('_result', '')
+    translation_file = result_dir / f"{base_name}_translated_{lang}.json"
+
+    if not translation_file.exists():
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND,
+            detail=f"Translation for language '{lang}' not found"
+        )
+
+    # Delete file
+    translation_file.unlink()
+    logger.info(f"Deleted translation {lang} for task {task_id}")
+
+    return None
diff --git a/backend/app/schemas/__init__.py b/backend/app/schemas/__init__.py
index fae4160..f4036e5 100644
--- a/backend/app/schemas/__init__.py
+++ b/backend/app/schemas/__init__.py
@@ -13,6 +13,16 @@ from app.schemas.task import (
     TaskStatsResponse,
     TaskStatusEnum,
 )
+from app.schemas.translation import (
+    TranslationStatusEnum,
+    TranslationRequest,
+    TranslationProgress,
+    TranslationStatusResponse,
+    TranslationStartResponse,
+    TranslationStatistics,
+    TranslationResultResponse,
+    TranslationListResponse,
+)
 
 __all__ = [
     # Auth
@@ -27,4 +37,13 @@ __all__ = [
     "TaskListResponse",
     "TaskStatsResponse",
     "TaskStatusEnum",
+    # Translation
+    "TranslationStatusEnum",
+    "TranslationRequest",
+    "TranslationProgress",
+    "TranslationStatusResponse",
+    "TranslationStartResponse",
+    "TranslationStatistics",
+    "TranslationResultResponse",
+    "TranslationListResponse",
 ]
diff --git a/backend/app/schemas/translation.py b/backend/app/schemas/translation.py
new file mode 100644
index 0000000..11c2902
--- /dev/null
+++ b/backend/app/schemas/translation.py
@@ -0,0 +1,163 @@
+"""
+Tool_OCR - Translation Schemas
+Pydantic models for document translation feature (DIFY API)
+"""
+
+from typing import Optional, List, Dict, Any, Tuple
+from datetime import datetime
+from pydantic import BaseModel, Field
+from enum import Enum
+from dataclasses import dataclass
+
+
+class TranslationStatusEnum(str, Enum):
+    """Translation job status enumeration"""
+    PENDING = "pending"
+    TRANSLATING = "translating"
+    COMPLETED = "completed"
+    FAILED = "failed"
+
+
+class TargetLanguageEnum(str, Enum):
+    """Supported target languages for translation."""
+    ENGLISH = "en"
+    JAPANESE = "ja"
+    KOREAN = "ko"
+    CHINESE_SIMPLIFIED = "zh-CN"
+    CHINESE_TRADITIONAL = "zh-TW"
+    GERMAN = "de"
+    FRENCH = "fr"
+    SPANISH = "es"
+    PORTUGUESE = "pt"
+    ITALIAN = "it"
+    RUSSIAN = "ru"
+    VIETNAMESE = "vi"
+    THAI = "th"
+
+
+class TranslationRequest(BaseModel):
+    """Request model for starting a translation job"""
+    target_lang: str = Field(
+        ...,
+        description="Target language code (e.g., 'en', 'ja', 'zh-TW')"
+    )
+    source_lang: str = Field(
+        default="auto",
+        description="Source language code, 'auto' for automatic detection"
+    )
+
+
+class TranslationProgress(BaseModel):
+    """Progress information for ongoing translation"""
+    current_element: int = Field(default=0, description="Current element being translated")
+    total_elements: int = Field(default=0, description="Total elements to translate")
+    percentage: float = Field(default=0.0, description="Progress percentage (0-100)")
+
+
+class TranslationStatusResponse(BaseModel):
+    """Response model for translation status query"""
+    task_id: str = Field(..., description="Task ID")
+    status: TranslationStatusEnum = Field(..., description="Current translation status")
+    target_lang: str = Field(..., description="Target language")
+    progress: Optional[TranslationProgress] = Field(
+        default=None,
+        description="Progress information when translating"
+    )
+    error_message: Optional[str] = Field(
+        default=None,
+        description="Error message if translation failed"
+    )
+    started_at: Optional[datetime] = Field(default=None, description="Translation start time")
+    completed_at: Optional[datetime] = Field(default=None, description="Translation completion time")
+
+
+class TranslationStartResponse(BaseModel):
+    """Response model for starting a translation job"""
+    task_id: str = Field(..., description="Task ID")
+    status: TranslationStatusEnum = Field(..., description="Initial status")
+    target_lang: str = Field(..., description="Target language")
+    message: str = Field(..., description="Status message")
+
+
+class TranslationStatistics(BaseModel):
+    """Statistics for completed translation"""
+    total_elements: int = Field(default=0, description="Total elements in document")
+    translated_elements: int = Field(default=0, description="Successfully translated elements")
+    skipped_elements: int = Field(default=0, description="Skipped elements (images, etc.)")
+    total_characters: int = Field(default=0, description="Total characters translated")
+    processing_time_seconds: float = Field(default=0.0, description="Translation duration")
+    total_tokens: int = Field(default=0, description="Total API tokens used")
+
+
+class TranslationResultResponse(BaseModel):
+    """Response model for translation result"""
+    schema_version: str = Field(default="1.0.0", description="Schema version")
+    source_document: str = Field(..., description="Source document filename")
+    source_lang: str = Field(..., description="Source language (detected or specified)")
+    target_lang: str = Field(..., description="Target language")
+    provider: str = Field(default="dify", description="Translation provider")
+    translated_at: datetime = Field(..., description="Translation timestamp")
+    statistics: TranslationStatistics = Field(..., description="Translation statistics")
+    translations: Dict[str, Any] = Field(
+        ...,
+        description="Translations dict mapping element_id to translated content"
+    )
+
+
+class TranslationListItem(BaseModel):
+    """Item in translation list response"""
+    target_lang: str = Field(..., description="Target language")
+    translated_at: datetime = Field(..., description="Translation timestamp")
+    provider: str = Field(default="dify", description="Translation provider")
+    statistics: TranslationStatistics = Field(..., description="Translation statistics")
+    file_path: str = Field(..., description="Path to translation JSON file")
+
+
+class TranslationListResponse(BaseModel):
+    """Response model for listing available translations"""
+    task_id: str = Field(..., description="Task ID")
+    translations: List[TranslationListItem] = Field(
+        default_factory=list,
+        description="Available translations"
+    )
+
+
+# Dataclasses for internal use
+
+@dataclass
+class TranslatableItem:
+    """Internal representation of a translatable element"""
+    element_id: str
+    content: str
+    element_type: str  # 'text', 'title', 'header', etc. or 'table_cell'
+    page_number: int = 1
+    cell_position: Optional[Tuple[int, int]] = None  # (row, col) for table cells
+
+    def __post_init__(self):
+        # Clean content - remove excessive whitespace
+        if self.content:
+            self.content = ' '.join(self.content.split())
+
+
+@dataclass
+class TranslatedItem:
+    """Internal representation of a translated element"""
+    element_id: str
+    original_content: str
+    translated_content: str
+    element_type: str
+    cell_position: Optional[Tuple[int, int]] = None
+
+
+@dataclass
+class TranslationJobState:
+    """Internal state for a translation job"""
+    task_id: str
+    target_lang: str
+    source_lang: str
+    status: TranslationStatusEnum
+    progress: TranslationProgress
+    error_message: Optional[str] = None
+    started_at: Optional[datetime] = None
+    completed_at: Optional[datetime] = None
+    result_file_path: Optional[str] = None
diff --git a/backend/app/services/dify_client.py b/backend/app/services/dify_client.py
new file mode 100644
index 0000000..bfd00d7
--- /dev/null
+++ b/backend/app/services/dify_client.py
@@ -0,0 +1,332 @@
+"""
+Tool_OCR - DIFY AI Client
+HTTP client for DIFY translation API with batch support
+"""
+
+import asyncio
+import logging
+import re
+import time
+from dataclasses import dataclass, field
+from typing import Dict, List, Optional
+
+import httpx
+
+logger = logging.getLogger(__name__)
+
+# DIFY API Configuration
+DIFY_BASE_URL = "https://dify.theaken.com/v1"
+DIFY_API_KEY = "app-YOPrF2ro5fshzMkCZviIuUJd"
+DIFY_TIMEOUT = 120.0  # seconds (increased for batch)
+DIFY_MAX_RETRIES = 3
+
+# Batch translation limits
+# Conservative limits to avoid gateway timeouts
+# DIFY server may have processing time limits
+MAX_BATCH_CHARS = 5000
+MAX_BATCH_ITEMS = 20
+
+# Language name mapping
+LANGUAGE_NAMES = {
+    "en": "English",
+    "zh-TW": "Traditional Chinese",
+    "zh-CN": "Simplified Chinese",
+    "ja": "Japanese",
+    "ko": "Korean",
+    "de": "German",
+    "fr": "French",
+    "es": "Spanish",
+    "pt": "Portuguese",
+    "it": "Italian",
+    "ru": "Russian",
+    "vi": "Vietnamese",
+    "th": "Thai",
+}
+
+
+@dataclass
+class TranslationResponse:
+    """Response from DIFY translation API"""
+    translated_text: str
+    total_tokens: int
+    latency: float
+    conversation_id: str
+
+
+@dataclass
+class BatchTranslationResponse:
+    """Response from DIFY batch translation API"""
+    translations: Dict[int, str]  # marker_id -> translated_text
+    total_tokens: int
+    latency: float
+    conversation_id: str
+    missing_markers: List[int] = field(default_factory=list)
+
+
+class DifyTranslationError(Exception):
+    """Error during DIFY API translation"""
+    pass
+
+
+class DifyClient:
+    """
+    Client for DIFY AI translation API.
+
+    Features:
+    - Single and batch translation
+    - Blocking mode API calls
+    - Automatic retry with exponential backoff
+    - Token and latency tracking
+    """
+
+    def __init__(
+        self,
+        base_url: str = DIFY_BASE_URL,
+        api_key: str = DIFY_API_KEY,
+        timeout: float = DIFY_TIMEOUT,
+        max_retries: int = DIFY_MAX_RETRIES
+    ):
+        self.base_url = base_url
+        self.api_key = api_key
+        self.timeout = timeout
+        self.max_retries = max_retries
+        self._total_tokens = 0
+        self._total_requests = 0
+
+    def _get_language_name(self, lang_code: str) -> str:
+        """Convert language code to full name for prompt"""
+        return LANGUAGE_NAMES.get(lang_code, lang_code)
+
+    def _build_prompt(self, text: str, target_lang: str) -> str:
+        """Build translation prompt for single text"""
+        lang_name = self._get_language_name(target_lang)
+        return (
+            f"Translate the following text to {lang_name}.\n"
+            f"Return ONLY the translated text, no explanations.\n\n"
+            f"{text}"
+        )
+
+    def _build_batch_prompt(self, texts: List[str], target_lang: str) -> str:
+        """
+        Build batch translation prompt with numbered markers.
+
+        Format:
+        Translate the following texts to {Language}.
+        Each text is marked with [N]. Return translations in the same format.
+        Return ONLY the translations with their markers, no explanations.
+
+        [1] First text
+        [2] Second text
+        ...
+        """
+        lang_name = self._get_language_name(target_lang)
+
+        # Build numbered text list
+        numbered_texts = []
+        for i, text in enumerate(texts, start=1):
+            # Clean text - remove newlines within each item to avoid parsing issues
+            clean_text = ' '.join(text.split())
+            numbered_texts.append(f"[{i}] {clean_text}")
+
+        texts_block = "\n".join(numbered_texts)
+
+        prompt = (
+            f"Translate the following texts to {lang_name}.\n"
+            f"Each text is marked with [N]. Return translations in the same format.\n"
+            f"Return ONLY the translations with their markers, no explanations.\n\n"
+            f"{texts_block}"
+        )
+
+        return prompt
+
+    def _parse_batch_response(self, response_text: str, expected_count: int) -> Dict[int, str]:
+        """
+        Parse batch translation response with numbered markers.
+
+        Expected format:
+        [1] 翻譯文字一
+        [2] 翻譯文字二
+        ...
+
+        Returns:
+            Dict mapping marker number to translated text
+        """
+        translations = {}
+
+        # Pattern to match [N] followed by text until next [N] or end
+        # Use DOTALL to match across lines, but be careful with greedy matching
+        pattern = r'\[(\d+)\]\s*(.+?)(?=\[\d+\]|$)'
+        matches = re.findall(pattern, response_text, re.DOTALL)
+
+        for match in matches:
+            try:
+                marker_id = int(match[0])
+                text = match[1].strip()
+                if text:
+                    translations[marker_id] = text
+            except (ValueError, IndexError):
+                continue
+
+        return translations
+
+    def _call_api(self, prompt: str, user_id: str) -> dict:
+        """Make API call to DIFY with retry logic"""
+        payload = {
+            "inputs": {},
+            "query": prompt,
+            "response_mode": "blocking",
+            "conversation_id": "",
+            "user": user_id
+        }
+
+        headers = {
+            "Authorization": f"Bearer {self.api_key}",
+            "Content-Type": "application/json"
+        }
+
+        last_error = None
+
+        for attempt in range(self.max_retries):
+            try:
+                with httpx.Client(timeout=self.timeout) as client:
+                    response = client.post(
+                        f"{self.base_url}/chat-messages",
+                        json=payload,
+                        headers=headers
+                    )
+
+                    if response.status_code != 200:
+                        raise DifyTranslationError(
+                            f"API returned status {response.status_code}: {response.text}"
+                        )
+
+                    return response.json()
+
+            except httpx.TimeoutException as e:
+                last_error = e
+                logger.warning(f"DIFY API timeout (attempt {attempt + 1}/{self.max_retries})")
+
+            except httpx.RequestError as e:
+                last_error = e
+                logger.warning(f"DIFY API request error (attempt {attempt + 1}/{self.max_retries}): {e}")
+
+            except Exception as e:
+                last_error = e
+                logger.warning(f"DIFY API error (attempt {attempt + 1}/{self.max_retries}): {e}")
+
+            # Exponential backoff
+            if attempt < self.max_retries - 1:
+                wait_time = 2 ** attempt
+                logger.info(f"Retrying in {wait_time}s...")
+                time.sleep(wait_time)
+
+        raise DifyTranslationError(f"API call failed after {self.max_retries} attempts: {last_error}")
+
+    def translate(
+        self,
+        text: str,
+        target_lang: str,
+        user_id: str = "tool-ocr"
+    ) -> TranslationResponse:
+        """
+        Translate single text using DIFY API.
+
+        Args:
+            text: Text to translate
+            target_lang: Target language code (e.g., 'en', 'zh-TW')
+            user_id: User identifier for tracking
+
+        Returns:
+            TranslationResponse with translated text and metadata
+        """
+        prompt = self._build_prompt(text, target_lang)
+        data = self._call_api(prompt, user_id)
+
+        # Extract response fields
+        translated_text = data.get("answer", "")
+        usage = data.get("metadata", {}).get("usage", {})
+
+        self._total_tokens += usage.get("total_tokens", 0)
+        self._total_requests += 1
+
+        return TranslationResponse(
+            translated_text=translated_text,
+            total_tokens=usage.get("total_tokens", 0),
+            latency=usage.get("latency", 0.0),
+            conversation_id=data.get("conversation_id", "")
+        )
+
+    def translate_batch(
+        self,
+        texts: List[str],
+        target_lang: str,
+        user_id: str = "tool-ocr"
+    ) -> BatchTranslationResponse:
+        """
+        Translate multiple texts in a single API call.
+
+        Args:
+            texts: List of texts to translate
+            target_lang: Target language code
+            user_id: User identifier for tracking
+
+        Returns:
+            BatchTranslationResponse with translations dict and metadata
+        """
+        if not texts:
+            return BatchTranslationResponse(
+                translations={},
+                total_tokens=0,
+                latency=0.0,
+                conversation_id=""
+            )
+
+        prompt = self._build_batch_prompt(texts, target_lang)
+
+        logger.debug(f"Batch translation: {len(texts)} items, ~{len(prompt)} chars")
+
+        data = self._call_api(prompt, user_id)
+
+        # Extract and parse response
+        answer = data.get("answer", "")
+        usage = data.get("metadata", {}).get("usage", {})
+
+        translations = self._parse_batch_response(answer, len(texts))
+
+        # Check for missing markers
+        missing_markers = []
+        for i in range(1, len(texts) + 1):
+            if i not in translations:
+                missing_markers.append(i)
+                logger.warning(f"Missing translation for marker [{i}]")
+
+        self._total_tokens += usage.get("total_tokens", 0)
+        self._total_requests += 1
+
+        return BatchTranslationResponse(
+            translations=translations,
+            total_tokens=usage.get("total_tokens", 0),
+            latency=usage.get("latency", 0.0),
+            conversation_id=data.get("conversation_id", ""),
+            missing_markers=missing_markers
+        )
+
+    def get_stats(self) -> dict:
+        """Get client statistics"""
+        return {
+            "total_tokens": self._total_tokens,
+            "total_requests": self._total_requests,
+            "base_url": self.base_url,
+        }
+
+
+# Global singleton
+_dify_client: Optional[DifyClient] = None
+
+
+def get_dify_client() -> DifyClient:
+    """Get the global DifyClient instance"""
+    global _dify_client
+    if _dify_client is None:
+        _dify_client = DifyClient()
+    return _dify_client
diff --git a/backend/app/services/translation_service.py b/backend/app/services/translation_service.py
new file mode 100644
index 0000000..12abebd
--- /dev/null
+++ b/backend/app/services/translation_service.py
@@ -0,0 +1,490 @@
+"""
+Tool_OCR - Translation Service
+Document translation using DIFY AI API with batch processing
+"""
+
+import json
+import logging
+import threading
+import time
+from dataclasses import dataclass, field
+from datetime import datetime
+from pathlib import Path
+from typing import Any, Dict, List, Optional, Tuple
+
+from app.schemas.translation import (
+    TranslatableItem,
+    TranslatedItem,
+    TranslationJobState,
+    TranslationProgress,
+    TranslationStatusEnum,
+)
+from app.services.dify_client import (
+    DifyClient,
+    DifyTranslationError,
+    get_dify_client,
+    MAX_BATCH_CHARS,
+    MAX_BATCH_ITEMS,
+)
+
+logger = logging.getLogger(__name__)
+
+# Element types that should be translated
+TRANSLATABLE_TEXT_TYPES = {'text', 'title', 'header', 'footer', 'paragraph', 'footnote'}
+TABLE_TYPE = 'table'
+SKIP_TYPES = {'page_number', 'image', 'chart', 'logo', 'reference'}
+
+
+@dataclass
+class TranslationBatch:
+    """A batch of items to translate together"""
+    items: List[TranslatableItem] = field(default_factory=list)
+    total_chars: int = 0
+
+    def can_add(self, item: TranslatableItem) -> bool:
+        """Check if item can be added to this batch"""
+        item_chars = len(item.content)
+        return (
+            len(self.items) < MAX_BATCH_ITEMS and
+            self.total_chars + item_chars <= MAX_BATCH_CHARS
+        )
+
+    def add(self, item: TranslatableItem):
+        """Add item to batch"""
+        self.items.append(item)
+        self.total_chars += len(item.content)
+
+
+class TranslationService:
+    """
+    Main translation service for document translation using DIFY AI.
+
+    Features:
+    - Extract translatable elements from UnifiedDocument
+    - Batch translation via DIFY API (efficient)
+    - Fallback to single-item translation for failures
+    - Translation JSON generation
+    - Progress tracking
+    """
+
+    def __init__(self, dify_client: Optional[DifyClient] = None):
+        self.dify_client = dify_client or get_dify_client()
+        self._active_jobs: Dict[str, TranslationJobState] = {}
+        self._jobs_lock = threading.Lock()
+        self._total_tokens = 0
+        self._total_latency = 0.0
+
+    def extract_translatable_elements(
+        self,
+        result_json: Dict
+    ) -> Tuple[List[TranslatableItem], int]:
+        """
+        Extract all translatable elements from a result JSON.
+
+        Args:
+            result_json: UnifiedDocument JSON data
+
+        Returns:
+            Tuple of (list of TranslatableItem, total element count)
+        """
+        items = []
+        total_elements = 0
+
+        for page in result_json.get('pages', []):
+            page_number = page.get('page_number', 1)
+
+            for elem in page.get('elements', []):
+                total_elements += 1
+                elem_type = elem.get('type', '')
+                elem_id = elem.get('element_id', '')
+                content = elem.get('content')
+
+                # Skip non-translatable types
+                if elem_type in SKIP_TYPES:
+                    continue
+
+                # Handle text elements
+                if elem_type in TRANSLATABLE_TEXT_TYPES and isinstance(content, str):
+                    text = content.strip()
+                    if text:  # Skip empty content
+                        items.append(TranslatableItem(
+                            element_id=elem_id,
+                            content=text,
+                            element_type=elem_type,
+                            page_number=page_number
+                        ))
+
+                # Handle table elements
+                elif elem_type == TABLE_TYPE and isinstance(content, dict):
+                    cells = content.get('cells', [])
+                    for cell in cells:
+                        cell_content = cell.get('content', '')
+                        if isinstance(cell_content, str) and cell_content.strip():
+                            row = cell.get('row', 0)
+                            col = cell.get('col', 0)
+                            items.append(TranslatableItem(
+                                element_id=elem_id,
+                                content=cell_content.strip(),
+                                element_type='table_cell',
+                                page_number=page_number,
+                                cell_position=(row, col)
+                            ))
+
+        logger.info(
+            f"Extracted {len(items)} translatable items from {total_elements} elements"
+        )
+        return items, total_elements
+
+    def create_batches(self, items: List[TranslatableItem]) -> List[TranslationBatch]:
+        """
+        Create translation batches from items based on character limits.
+
+        Args:
+            items: List of TranslatableItem
+
+        Returns:
+            List of TranslationBatch
+        """
+        batches = []
+        current_batch = TranslationBatch()
+
+        for item in items:
+            if current_batch.can_add(item):
+                current_batch.add(item)
+            else:
+                # Save current batch and start new one
+                if current_batch.items:
+                    batches.append(current_batch)
+                current_batch = TranslationBatch()
+                current_batch.add(item)
+
+        # Don't forget the last batch
+        if current_batch.items:
+            batches.append(current_batch)
+
+        logger.info(
+            f"Created {len(batches)} batches from {len(items)} items "
+            f"(max {MAX_BATCH_CHARS} chars, max {MAX_BATCH_ITEMS} items per batch)"
+        )
+
+        return batches
+
+    def translate_batch(
+        self,
+        batch: TranslationBatch,
+        target_lang: str,
+        user_id: str
+    ) -> List[TranslatedItem]:
+        """
+        Translate a batch of items using DIFY API.
+
+        Args:
+            batch: TranslationBatch to translate
+            target_lang: Target language code
+            user_id: User identifier for tracking
+
+        Returns:
+            List of TranslatedItem
+        """
+        if not batch.items:
+            return []
+
+        # Extract texts in order
+        texts = [item.content for item in batch.items]
+
+        try:
+            response = self.dify_client.translate_batch(
+                texts=texts,
+                target_lang=target_lang,
+                user_id=user_id
+            )
+
+            self._total_tokens += response.total_tokens
+            self._total_latency += response.latency
+
+            # Map translations back to items
+            translated_items = []
+            for idx, item in enumerate(batch.items):
+                marker_id = idx + 1  # Markers are 1-indexed
+
+                if marker_id in response.translations:
+                    translated_content = response.translations[marker_id]
+                else:
+                    # Missing translation - use original
+                    logger.warning(f"Missing translation for {item.element_id}, using original")
+                    translated_content = item.content
+
+                translated_items.append(TranslatedItem(
+                    element_id=item.element_id,
+                    original_content=item.content,
+                    translated_content=translated_content,
+                    element_type=item.element_type,
+                    cell_position=item.cell_position
+                ))
+
+            return translated_items
+
+        except DifyTranslationError as e:
+            logger.error(f"Batch translation failed: {e}")
+            # Return items with original content on failure
+            return [
+                TranslatedItem(
+                    element_id=item.element_id,
+                    original_content=item.content,
+                    translated_content=item.content,  # Keep original
+                    element_type=item.element_type,
+                    cell_position=item.cell_position
+                )
+                for item in batch.items
+            ]
+
+    def translate_item(
+        self,
+        item: TranslatableItem,
+        target_lang: str,
+        user_id: str
+    ) -> TranslatedItem:
+        """
+        Translate a single item using DIFY API (fallback for batch failures).
+
+        Args:
+            item: TranslatableItem to translate
+            target_lang: Target language code
+            user_id: User identifier for tracking
+
+        Returns:
+            TranslatedItem with translation result
+        """
+        try:
+            response = self.dify_client.translate(
+                text=item.content,
+                target_lang=target_lang,
+                user_id=user_id
+            )
+
+            self._total_tokens += response.total_tokens
+            self._total_latency += response.latency
+
+            return TranslatedItem(
+                element_id=item.element_id,
+                original_content=item.content,
+                translated_content=response.translated_text,
+                element_type=item.element_type,
+                cell_position=item.cell_position
+            )
+
+        except DifyTranslationError as e:
+            logger.error(f"Translation failed for {item.element_id}: {e}")
+            # Return original content on failure
+            return TranslatedItem(
+                element_id=item.element_id,
+                original_content=item.content,
+                translated_content=item.content,  # Keep original
+                element_type=item.element_type,
+                cell_position=item.cell_position
+            )
+
+    def build_translation_result(
+        self,
+        translated_items: List[TranslatedItem],
+        source_document: str,
+        source_lang: str,
+        target_lang: str,
+        total_elements: int,
+        processing_time: float,
+        batch_count: int
+    ) -> Dict:
+        """
+        Build the translation result JSON structure.
+
+        Args:
+            translated_items: List of TranslatedItem
+            source_document: Source document filename
+            source_lang: Source language
+            target_lang: Target language
+            total_elements: Total elements in document
+            processing_time: Processing time in seconds
+            batch_count: Number of batches used
+
+        Returns:
+            Translation result dictionary
+        """
+        # Build translations dict
+        translations: Dict[str, Any] = {}
+        total_chars = 0
+
+        for item in translated_items:
+            total_chars += len(item.translated_content)
+
+            if item.element_type == 'table_cell':
+                # Group table cells by element_id
+                if item.element_id not in translations:
+                    translations[item.element_id] = {'cells': []}
+
+                translations[item.element_id]['cells'].append({
+                    'row': item.cell_position[0] if item.cell_position else 0,
+                    'col': item.cell_position[1] if item.cell_position else 0,
+                    'content': item.translated_content
+                })
+            else:
+                translations[item.element_id] = item.translated_content
+
+        # Build statistics
+        translated_element_ids = set(item.element_id for item in translated_items)
+        skipped = total_elements - len(translated_element_ids)
+
+        result = {
+            'schema_version': '1.0.0',
+            'source_document': source_document,
+            'source_lang': source_lang,
+            'target_lang': target_lang,
+            'provider': 'dify',
+            'translated_at': datetime.utcnow().isoformat() + 'Z',
+            'statistics': {
+                'total_elements': total_elements,
+                'translated_elements': len(translated_element_ids),
+                'skipped_elements': skipped,
+                'total_characters': total_chars,
+                'processing_time_seconds': round(processing_time, 2),
+                'total_tokens': self._total_tokens,
+                'batch_count': batch_count
+            },
+            'translations': translations
+        }
+
+        return result
+
+    def translate_document(
+        self,
+        task_id: str,
+        result_json_path: Path,
+        target_lang: str,
+        source_lang: str = 'auto',
+        progress_callback: Optional[callable] = None
+    ) -> Tuple[bool, Optional[Path], Optional[str]]:
+        """
+        Translate a document using batch processing and save the result.
+
+        Args:
+            task_id: Task ID
+            result_json_path: Path to source result.json
+            target_lang: Target language (e.g., 'en', 'zh-TW')
+            source_lang: Source language ('auto' for detection)
+            progress_callback: Optional callback(progress: TranslationProgress)
+
+        Returns:
+            Tuple of (success, output_path, error_message)
+        """
+        start_time = time.time()
+        self._total_tokens = 0
+        self._total_latency = 0.0
+
+        logger.info(
+            f"Starting translation: task_id={task_id}, target={target_lang}"
+        )
+
+        try:
+            # Load source JSON
+            with open(result_json_path, 'r', encoding='utf-8') as f:
+                result_json = json.load(f)
+
+            source_document = result_json.get('metadata', {}).get('filename', 'unknown')
+
+            # Extract translatable elements
+            items, total_elements = self.extract_translatable_elements(result_json)
+
+            if not items:
+                logger.warning("No translatable elements found")
+                return False, None, "No translatable elements found"
+
+            # Create batches
+            batches = self.create_batches(items)
+
+            # Update initial progress
+            if progress_callback:
+                progress_callback(TranslationProgress(
+                    total_elements=len(items)
+                ))
+
+            # Translate each batch
+            all_translated: List[TranslatedItem] = []
+            user_id = f"tool-ocr-{task_id}"
+            processed_items = 0
+
+            for batch_idx, batch in enumerate(batches):
+                logger.info(
+                    f"Translating batch {batch_idx + 1}/{len(batches)} "
+                    f"({len(batch.items)} items, {batch.total_chars} chars)"
+                )
+
+                translated = self.translate_batch(batch, target_lang, user_id)
+                all_translated.extend(translated)
+                processed_items += len(batch.items)
+
+                # Update progress
+                if progress_callback:
+                    progress_callback(TranslationProgress(
+                        current_element=processed_items,
+                        total_elements=len(items),
+                        percentage=(processed_items / len(items)) * 100
+                    ))
+
+            # Build result
+            processing_time = time.time() - start_time
+            result = self.build_translation_result(
+                translated_items=all_translated,
+                source_document=source_document,
+                source_lang=source_lang,
+                target_lang=target_lang,
+                total_elements=total_elements,
+                processing_time=processing_time,
+                batch_count=len(batches)
+            )
+
+            # Save result
+            output_filename = result_json_path.stem.replace('_result', '')
+            output_path = result_json_path.parent / f"{output_filename}_translated_{target_lang}.json"
+
+            with open(output_path, 'w', encoding='utf-8') as f:
+                json.dump(result, f, ensure_ascii=False, indent=2)
+
+            logger.info(
+                f"Translation completed: {len(all_translated)} items in {len(batches)} batches, "
+                f"{processing_time:.2f}s, {self._total_tokens} tokens, "
+                f"saved to {output_path}"
+            )
+
+            return True, output_path, None
+
+        except Exception as e:
+            logger.error(f"Translation failed: {e}")
+            import traceback
+            traceback.print_exc()
+            return False, None, str(e)
+
+    def get_job_state(self, task_id: str) -> Optional[TranslationJobState]:
+        """Get the current state of a translation job"""
+        with self._jobs_lock:
+            return self._active_jobs.get(task_id)
+
+    def set_job_state(self, task_id: str, state: TranslationJobState):
+        """Set the state of a translation job"""
+        with self._jobs_lock:
+            self._active_jobs[task_id] = state
+
+    def remove_job_state(self, task_id: str):
+        """Remove a translation job state"""
+        with self._jobs_lock:
+            self._active_jobs.pop(task_id, None)
+
+
+# Global singleton
+_translation_service: Optional[TranslationService] = None
+
+
+def get_translation_service() -> TranslationService:
+    """Get the global TranslationService instance"""
+    global _translation_service
+    if _translation_service is None:
+        _translation_service = TranslationService()
+    return _translation_service
diff --git a/backend/tests/test_translation_real.py b/backend/tests/test_translation_real.py
new file mode 100644
index 0000000..53fbe0f
--- /dev/null
+++ b/backend/tests/test_translation_real.py
@@ -0,0 +1,138 @@
+#!/usr/bin/env python3
+"""
+Test translation service with DIFY API using real OCR results from storage/results/
+"""
+import json
+import pytest
+from pathlib import Path
+
+from app.services.dify_client import DifyClient, get_dify_client
+from app.services.translation_service import TranslationService, get_translation_service
+
+# Real task IDs with their result files
+REAL_TASKS = [
+    ("ca2b59a3-3362-4678-954f-cf0a9bcc152e", "img3_result.json"),
+    ("8ab2f24d-992b-46a2-87dc-2e024e006ac7", "img1_result.json"),
+    ("1c94bfbf-9391-444c-bebf-ae22fa3dad32", "edit_result.json"),
+    ("c85fff69-9ddb-40b8-8a9b-ebb513c60f05", "scan_result.json"),
+    ("0088e960-7b61-4cdf-bfe5-956960b00dd1", "scan2_result.json"),
+    ("8eedd9ed-7aad-46d5-93ca-951352c954b9", "ppt_result.json"),
+    ("992156c5-72b4-4e3d-8d43-cbb15f23e630", "edit3_result.json"),
+    ("1484ba43-7484-4326-95a7-1544b181e9e8", "edit2_result.json"),
+    ("e9a16bba-7d37-42f4-84c8-6624cb58fe19", "img2_result.json"),
+]
+
+RESULTS_DIR = Path(__file__).parent.parent / "storage" / "results"
+
+
+@pytest.fixture
+def dify_client():
+    """Get DIFY client instance"""
+    return get_dify_client()
+
+
+@pytest.fixture
+def translation_service():
+    """Get translation service instance"""
+    return get_translation_service()
+
+
+class TestDifyClient:
+    """Test DIFY API client"""
+
+    def test_client_initialization(self, dify_client):
+        """Test client can be initialized"""
+        assert dify_client is not None
+        assert dify_client.api_key is not None
+
+    def test_simple_translation(self, dify_client):
+        """Test simple translation via DIFY API"""
+        text = "Hello, this is a test."
+        response = dify_client.translate(text, "zh-TW")
+
+        assert response.translated_text is not None
+        assert len(response.translated_text) > 0
+        assert response.total_tokens > 0
+
+        print(f"\nOriginal: {text}")
+        print(f"Translated: {response.translated_text}")
+        print(f"Tokens: {response.total_tokens}, Latency: {response.latency:.2f}s")
+
+
+class TestTranslationServiceExtraction:
+    """Test element extraction"""
+
+    def test_service_initialization(self, translation_service):
+        """Test service can be initialized"""
+        assert translation_service is not None
+        assert translation_service.dify_client is not None
+
+    @pytest.mark.parametrize("task_id,result_file", REAL_TASKS)
+    def test_extract_translatable_elements(self, translation_service, task_id, result_file):
+        """Test extracting translatable elements from real OCR results"""
+        result_path = RESULTS_DIR / task_id / result_file
+        if not result_path.exists():
+            pytest.skip(f"Result file not found: {result_path}")
+
+        with open(result_path, 'r', encoding='utf-8') as f:
+            ocr_result = json.load(f)
+
+        elements, total_count = translation_service.extract_translatable_elements(ocr_result)
+
+        # Verify extraction
+        assert isinstance(elements, list)
+        assert isinstance(total_count, int)
+        assert total_count >= 0
+
+        print(f"\nTask {task_id}:")
+        print(f"  Extracted {len(elements)} translatable elements")
+        print(f"  Total elements in document: {total_count}")
+
+        if elements:
+            first = elements[0]
+            assert hasattr(first, 'element_id')
+            assert hasattr(first, 'content')
+            assert hasattr(first, 'element_type')
+            print(f"  First element type: {first.element_type}")
+            print(f"  First element preview: {first.content[:50]}..." if len(first.content) > 50 else f"  First element: {first.content}")
+
+
+class TestTranslationExecution:
+    """Test actual translation via DIFY API"""
+
+    @pytest.mark.parametrize("task_id,result_file", REAL_TASKS[:2])  # Test only first 2
+    def test_translate_first_3_elements(self, translation_service, task_id, result_file):
+        """Test translating first 3 elements from a real OCR document"""
+        result_path = RESULTS_DIR / task_id / result_file
+        if not result_path.exists():
+            pytest.skip(f"Result file not found: {result_path}")
+
+        with open(result_path, 'r', encoding='utf-8') as f:
+            ocr_result = json.load(f)
+
+        elements, _ = translation_service.extract_translatable_elements(ocr_result)
+        if not elements:
+            pytest.skip("No translatable elements found")
+
+        # Translate first 3 elements only
+        elements_to_translate = elements[:3]
+
+        print(f"\n{task_id} translations:")
+        for i, elem in enumerate(elements_to_translate):
+            translated = translation_service.translate_item(elem, "en", f"test-{task_id}")
+
+            assert translated.translated_content is not None
+            assert len(translated.translated_content) > 0
+
+            orig_preview = elem.content[:30] + "..." if len(elem.content) > 30 else elem.content
+            trans_preview = translated.translated_content[:30] + "..." if len(translated.translated_content) > 30 else translated.translated_content
+            print(f"  [{i+1}] {orig_preview} -> {trans_preview}")
+
+        print(f"  Total tokens: {translation_service._total_tokens}")
+
+
+if __name__ == "__main__":
+    # Run extraction tests only (no API calls) by default
+    # pytest.main([__file__, "-v", "-k", "extraction", "--tb=short"])
+    # Run all tests including API calls
+    pytest.main([__file__, "-v", "--tb=short"])
diff --git a/frontend/src/pages/TaskDetailPage.tsx b/frontend/src/pages/TaskDetailPage.tsx
index 2c5b6ff..547d37e 100644
--- a/frontend/src/pages/TaskDetailPage.tsx
+++ b/frontend/src/pages/TaskDetailPage.tsx
@@ -1,7 +1,7 @@
-import { useMemo } from 'react'
+import { useMemo, useState, useEffect } from 'react'
 import { useParams, useNavigate } from 'react-router-dom'
 import { useTranslation } from 'react-i18next'
-import { useQuery } from '@tanstack/react-query'
+import { useQuery, useQueryClient } from '@tanstack/react-query'
 import { Button } from '@/components/ui/button'
 import { Card, CardContent, CardHeader, CardTitle } from '@/components/ui/card'
 import PDFViewer from '@/components/PDFViewer'
@@ -11,23 +11,23 @@ import {
   FileText,
   Download,
   AlertCircle,
-  TrendingUp,
   Clock,
   Layers,
   FileJson,
   Loader2,
   ArrowLeft,
   RefreshCw,
-  Cpu,
   FileSearch,
   Table2,
   Image,
   BarChart3,
   Database,
   Languages,
-  Globe
+  Globe,
+  CheckCircle,
+  Trash2
 } from 'lucide-react'
-import type { ProcessingTrack, ProcessingMetadata } from '@/types/apiV2'
+import type { ProcessingTrack, TranslationStatus, TranslationListItem } from '@/types/apiV2'
 import { Badge } from '@/components/ui/badge'
 import {
   Select,
@@ -36,12 +36,37 @@ import {
   SelectTrigger,
   SelectValue
 } from '@/components/ui/select'
+import { Progress } from '@/components/ui/progress'
+
+// Language options for translation
+const LANGUAGE_OPTIONS = [
+  { value: 'en', label: 'English' },
+  { value: 'ja', label: '日本語' },
+  { value: 'ko', label: '한국어' },
+  { value: 'zh-TW', label: '繁體中文' },
+  { value: 'zh-CN', label: '简体中文' },
+  { value: 'de', label: 'Deutsch' },
+  { value: 'fr', label: 'Français' },
+  { value: 'es', label: 'Español' },
+  { value: 'pt', label: 'Português' },
+  { value: 'it', label: 'Italiano' },
+  { value: 'ru', label: 'Русский' },
+  { value: 'vi', label: 'Tiếng Việt' },
+  { value: 'th', label: 'ไทย' },
+]
 
 export default function TaskDetailPage() {
   const { taskId } = useParams<{ taskId: string }>()
   const { t } = useTranslation()
   const navigate = useNavigate()
   const { toast } = useToast()
+  const queryClient = useQueryClient()
+
+  // Translation state
+  const [targetLang, setTargetLang] = useState('en')
+  const [isTranslating, setIsTranslating] = useState(false)
+  const [translationStatus, setTranslationStatus] = useState<TranslationStatus | null>(null)
+  const [translationProgress, setTranslationProgress] = useState(0)
 
   // Get task details
   const { data: taskDetail, isLoading, refetch } = useQuery({
@@ -66,6 +91,52 @@ export default function TaskDetailPage() {
     retry: false,
   })
 
+  // Get existing translations
+  const { data: translationList, refetch: refetchTranslations } = useQuery({
+    queryKey: ['translations', taskId],
+    queryFn: () => apiClientV2.listTranslations(taskId!),
+    enabled: !!taskId && taskDetail?.status === 'completed',
+    retry: false,
+  })
+
+  // Poll translation status when translating
+  useEffect(() => {
+    if (!isTranslating || !taskId) return
+
+    const pollInterval = setInterval(async () => {
+      try {
+        const status = await apiClientV2.getTranslationStatus(taskId)
+        setTranslationStatus(status.status)
+
+        if (status.progress) {
+          setTranslationProgress(status.progress.percentage)
+        }
+
+        if (status.status === 'completed') {
+          setIsTranslating(false)
+          setTranslationProgress(100)
+          toast({
+            title: '翻譯完成',
+            description: `文件已翻譯為 ${LANGUAGE_OPTIONS.find(l => l.value === targetLang)?.label || targetLang}`,
+            variant: 'success',
+          })
+          refetchTranslations()
+        } else if (status.status === 'failed') {
+          setIsTranslating(false)
+          toast({
+            title: '翻譯失敗',
+            description: status.error_message || '未知錯誤',
+            variant: 'destructive',
+          })
+        }
+      } catch (error) {
+        console.error('Failed to poll translation status:', error)
+      }
+    }, 2000)
+
+    return () => clearInterval(pollInterval)
+  }, [isTranslating, taskId, targetLang, toast, refetchTranslations])
+
   // Construct PDF URL for preview - memoize to prevent unnecessary reloads
   // Must be called unconditionally before any early returns (React hooks rule)
   const API_BASE_URL = import.meta.env.VITE_API_BASE_URL || 'http://localhost:8000'
@@ -178,6 +249,84 @@ export default function TaskDetailPage() {
     }
   }
 
+  const handleStartTranslation = async () => {
+    if (!taskId || isTranslating) return
+
+    try {
+      setIsTranslating(true)
+      setTranslationStatus('pending')
+      setTranslationProgress(0)
+
+      const response = await apiClientV2.startTranslation(taskId, {
+        target_lang: targetLang,
+        source_lang: 'auto',
+      })
+
+      if (response.status === 'completed') {
+        // Translation already exists
+        setIsTranslating(false)
+        setTranslationProgress(100)
+        toast({
+          title: '翻譯已存在',
+          description: response.message,
+          variant: 'success',
+        })
+        refetchTranslations()
+      } else {
+        setTranslationStatus(response.status)
+        toast({
+          title: '開始翻譯',
+          description: '翻譯任務已啟動，請稍候...',
+        })
+      }
+    } catch (error: any) {
+      setIsTranslating(false)
+      setTranslationStatus(null)
+      toast({
+        title: '啟動翻譯失敗',
+        description: error.response?.data?.detail || t('errors.networkError'),
+        variant: 'destructive',
+      })
+    }
+  }
+
+  const handleDownloadTranslation = async (lang: string) => {
+    if (!taskId) return
+    try {
+      await apiClientV2.downloadTranslation(taskId, lang)
+      toast({
+        title: '下載成功',
+        description: `翻譯結果 (${lang}) 已下載`,
+        variant: 'success',
+      })
+    } catch (error: any) {
+      toast({
+        title: '下載失敗',
+        description: error.response?.data?.detail || t('errors.networkError'),
+        variant: 'destructive',
+      })
+    }
+  }
+
+  const handleDeleteTranslation = async (lang: string) => {
+    if (!taskId) return
+    try {
+      await apiClientV2.deleteTranslation(taskId, lang)
+      toast({
+        title: '刪除成功',
+        description: `翻譯結果 (${lang}) 已刪除`,
+        variant: 'success',
+      })
+      refetchTranslations()
+    } catch (error: any) {
+      toast({
+        title: '刪除失敗',
+        description: error.response?.data?.detail || t('errors.networkError'),
+        variant: 'destructive',
+      })
+    }
+  }
+
   const getStatusBadge = (status: string) => {
     switch (status) {
       case 'completed':
@@ -191,6 +340,23 @@ export default function TaskDetailPage() {
     }
   }
 
+  const getTranslationStatusText = (status: TranslationStatus | null) => {
+    switch (status) {
+      case 'pending':
+        return '準備中...'
+      case 'loading_model':
+        return '載入翻譯模型...'
+      case 'translating':
+        return '翻譯中...'
+      case 'completed':
+        return '完成'
+      case 'failed':
+        return '失敗'
+      default:
+        return ''
+    }
+  }
+
   const formatDate = (dateStr: string) => {
     const date = new Date(dateStr)
     return date.toLocaleString('zh-TW')
@@ -350,42 +516,113 @@ export default function TaskDetailPage() {
         </Card>
       )}
 
-      {/* Translation Options (Coming Soon) */}
+      {/* Translation Options */}
       {isCompleted && (
-        <Card className="opacity-60">
+        <Card>
           <CardHeader>
             <CardTitle className="flex items-center gap-2">
               <Languages className="w-5 h-5" />
-              翻譯
-              <Badge variant="secondary" className="ml-2">即將推出</Badge>
+              文件翻譯
+              <Badge variant="secondary" className="ml-2">MADLAD-400</Badge>
             </CardTitle>
           </CardHeader>
-          <CardContent>
+          <CardContent className="space-y-4">
+            {/* Start New Translation */}
             <div className="flex flex-col md:flex-row items-start md:items-center gap-4">
               <div className="flex items-center gap-2">
                 <Globe className="w-4 h-4 text-muted-foreground" />
                 <span className="text-sm text-muted-foreground">目標語言：</span>
-                <Select disabled defaultValue="en">
+                <Select
+                  value={targetLang}
+                  onValueChange={setTargetLang}
+                  disabled={isTranslating}
+                >
                   <SelectTrigger className="w-40">
                     <SelectValue placeholder="選擇語言" />
                   </SelectTrigger>
                   <SelectContent>
-                    <SelectItem value="en">English</SelectItem>
-                    <SelectItem value="ja">日本語</SelectItem>
-                    <SelectItem value="ko">한국어</SelectItem>
-                    <SelectItem value="zh-TW">繁體中文</SelectItem>
-                    <SelectItem value="zh-CN">简体中文</SelectItem>
+                    {LANGUAGE_OPTIONS.map(lang => (
+                      <SelectItem key={lang.value} value={lang.value}>
+                        {lang.label}
+                      </SelectItem>
+                    ))}
                   </SelectContent>
                 </Select>
               </div>
-              <Button disabled className="gap-2">
-                <Languages className="w-4 h-4" />
-                開始翻譯
+              <Button
+                onClick={handleStartTranslation}
+                disabled={isTranslating}
+                className="gap-2"
+              >
+                {isTranslating ? (
+                  <Loader2 className="w-4 h-4 animate-spin" />
+                ) : (
+                  <Languages className="w-4 h-4" />
+                )}
+                {isTranslating ? getTranslationStatusText(translationStatus) : '開始翻譯'}
               </Button>
-              <p className="text-sm text-muted-foreground">
-                翻譯功能正在開發中，敬請期待。
-              </p>
             </div>
+
+            {/* Translation Progress */}
+            {isTranslating && (
+              <div className="space-y-2">
+                <div className="flex items-center justify-between text-sm">
+                  <span className="text-muted-foreground">{getTranslationStatusText(translationStatus)}</span>
+                  <span className="text-muted-foreground">{translationProgress.toFixed(0)}%</span>
+                </div>
+                <Progress value={translationProgress} className="h-2" />
+              </div>
+            )}
+
+            {/* Existing Translations */}
+            {translationList && translationList.translations.length > 0 && (
+              <div className="space-y-2">
+                <p className="text-sm font-medium text-muted-foreground">已完成的翻譯：</p>
+                <div className="space-y-2">
+                  {translationList.translations.map((item: TranslationListItem) => (
+                    <div
+                      key={item.target_lang}
+                      className="flex items-center justify-between p-3 bg-muted/50 rounded-lg"
+                    >
+                      <div className="flex items-center gap-3">
+                        <CheckCircle className="w-4 h-4 text-green-500" />
+                        <div>
+                          <span className="font-medium">
+                            {LANGUAGE_OPTIONS.find(l => l.value === item.target_lang)?.label || item.target_lang}
+                          </span>
+                          <span className="text-sm text-muted-foreground ml-2">
+                            ({item.statistics.translated_elements} 元素, {item.statistics.processing_time_seconds.toFixed(1)}s)
+                          </span>
+                        </div>
+                      </div>
+                      <div className="flex items-center gap-2">
+                        <Button
+                          variant="outline"
+                          size="sm"
+                          onClick={() => handleDownloadTranslation(item.target_lang)}
+                          className="gap-1"
+                        >
+                          <Download className="w-3 h-3" />
+                          下載
+                        </Button>
+                        <Button
+                          variant="ghost"
+                          size="sm"
+                          onClick={() => handleDeleteTranslation(item.target_lang)}
+                          className="gap-1 text-destructive hover:text-destructive"
+                        >
+                          <Trash2 className="w-3 h-3" />
+                        </Button>
+                      </div>
+                    </div>
+                  ))}
+                </div>
+              </div>
+            )}
+
+            <p className="text-xs text-muted-foreground">
+              使用 MADLAD-400-3B 模型進行翻譯，支援 450+ 種語言。首次翻譯需要載入模型（約 10-30 秒）。
+            </p>
           </CardContent>
         </Card>
       )}
diff --git a/frontend/src/services/apiV2.ts b/frontend/src/services/apiV2.ts
index e8b7def..0b03c5d 100644
--- a/frontend/src/services/apiV2.ts
+++ b/frontend/src/services/apiV2.ts
@@ -35,6 +35,11 @@ import type {
   DocumentAnalysisResponse,
   PreprocessingPreviewRequest,
   PreprocessingPreviewResponse,
+  TranslationRequest,
+  TranslationStartResponse,
+  TranslationStatusResponse,
+  TranslationListResponse,
+  TranslationResult,
 } from '@/types/apiV2'
 
 /**
@@ -613,6 +618,74 @@ class ApiClientV2 {
     )
     return response.data
   }
+
+  // ==================== Translation APIs ====================
+
+  /**
+   * Start a translation job
+   */
+  async startTranslation(taskId: string, request: TranslationRequest): Promise<TranslationStartResponse> {
+    const response = await this.client.post<TranslationStartResponse>(
+      `/translate/${taskId}`,
+      request
+    )
+    return response.data
+  }
+
+  /**
+   * Get translation status
+   */
+  async getTranslationStatus(taskId: string): Promise<TranslationStatusResponse> {
+    const response = await this.client.get<TranslationStatusResponse>(
+      `/translate/${taskId}/status`
+    )
+    return response.data
+  }
+
+  /**
+   * Get translation result
+   */
+  async getTranslationResult(taskId: string, lang: string): Promise<TranslationResult> {
+    const response = await this.client.get<TranslationResult>(
+      `/translate/${taskId}/result`,
+      { params: { lang } }
+    )
+    return response.data
+  }
+
+  /**
+   * Download translation result as JSON file
+   */
+  async downloadTranslation(taskId: string, lang: string): Promise<void> {
+    const response = await this.client.get(`/translate/${taskId}/result`, {
+      params: { lang },
+      responseType: 'blob',
+    })
+
+    const blob = new Blob([response.data], { type: 'application/json' })
+    const link = document.createElement('a')
+    link.href = window.URL.createObjectURL(blob)
+    link.download = `${taskId}_translated_${lang}.json`
+    link.click()
+    window.URL.revokeObjectURL(link.href)
+  }
+
+  /**
+   * List available translations for a task
+   */
+  async listTranslations(taskId: string): Promise<TranslationListResponse> {
+    const response = await this.client.get<TranslationListResponse>(
+      `/translate/${taskId}/translations`
+    )
+    return response.data
+  }
+
+  /**
+   * Delete a translation
+   */
+  async deleteTranslation(taskId: string, lang: string): Promise<void> {
+    await this.client.delete(`/translate/${taskId}/translations/${lang}`)
+  }
 }
 
 // Export singleton instance
diff --git a/frontend/src/types/apiV2.ts b/frontend/src/types/apiV2.ts
index 2f6027d..72f7240 100644
--- a/frontend/src/types/apiV2.ts
+++ b/frontend/src/types/apiV2.ts
@@ -307,3 +307,70 @@ export interface UserActivitySummary {
   categories: Record<string, number>
   recent_actions: AuditLog[]
 }
+
+// ==================== Translation Types ====================
+
+export type TranslationStatus = 'pending' | 'loading_model' | 'translating' | 'completed' | 'failed'
+
+export interface TranslationRequest {
+  target_lang: string
+  source_lang?: string
+}
+
+export interface TranslationProgress {
+  current_page: number
+  total_pages: number
+  current_element: number
+  total_elements: number
+  percentage: number
+}
+
+export interface TranslationStartResponse {
+  task_id: string
+  status: TranslationStatus
+  target_lang: string
+  message: string
+}
+
+export interface TranslationStatusResponse {
+  task_id: string
+  status: TranslationStatus
+  target_lang: string
+  progress: TranslationProgress | null
+  error_message: string | null
+  started_at: string | null
+  completed_at: string | null
+}
+
+export interface TranslationStatistics {
+  total_elements: number
+  translated_elements: number
+  skipped_elements: number
+  total_characters: number
+  processing_time_seconds: number
+}
+
+export interface TranslationListItem {
+  target_lang: string
+  translated_at: string
+  model: string
+  statistics: TranslationStatistics
+  file_path: string
+}
+
+export interface TranslationListResponse {
+  task_id: string
+  translations: TranslationListItem[]
+}
+
+export interface TranslationResult {
+  schema_version: string
+  source_document: string
+  source_lang: string
+  target_lang: string
+  model: string
+  model_version: string
+  translated_at: string
+  statistics: TranslationStatistics
+  translations: Record<string, any>
+}
diff --git a/openspec/changes/archive/2025-12-02-add-document-translation/design.md b/openspec/changes/archive/2025-12-02-add-document-translation/design.md
new file mode 100644
index 0000000..5b66bfb
--- /dev/null
+++ b/openspec/changes/archive/2025-12-02-add-document-translation/design.md
@@ -0,0 +1,265 @@
+# Design: Document Translation Feature
+
+## Context
+
+Tool_OCR processes documents through three tracks (Direct/OCR/Hybrid) and outputs UnifiedDocument JSON. Users need translation capability to convert extracted text into different languages while preserving document structure.
+
+### Constraints
+- Must use DIFY AI service for translation
+- API-based solution (no local model management)
+- Translation quality depends on DIFY's underlying model
+
+### Stakeholders
+- End users: Need translated documents
+- System: Simple HTTP-based integration
+
+## Goals / Non-Goals
+
+### Goals
+- Translate documents using DIFY AI API
+- Preserve document structure (element positions, formatting)
+- Support all three processing tracks with unified logic
+- Real-time progress feedback to users
+- Simple, maintainable API integration
+
+### Non-Goals
+- Local model inference (replaced by DIFY API)
+- GPU memory management (not needed)
+- Translation memory or glossary support
+- Concurrent translation processing
+
+## Decisions
+
+### Decision 1: Translation Provider
+
+**Choice**: DIFY AI Service (theaken.com)
+
+**Configuration**:
+- Base URL: `https://dify.theaken.com/v1`
+- Endpoint: `POST /chat-messages`
+- API Key: `app-YOPrF2ro5fshzMkCZviIuUJd`
+- Mode: Chat (Blocking response)
+
+**Rationale**:
+- High-quality cloud AI translation
+- No local model management required
+- No GPU memory concerns
+- Easy to maintain and update
+
+### Decision 2: Response Mode
+
+**Choice**: Blocking Mode
+
+**API Request Format**:
+```json
+{
+  "inputs": {},
+  "query": "Translate the following text to Chinese:\n\nHello world",
+  "response_mode": "blocking",
+  "conversation_id": "",
+  "user": "tool-ocr-{task_id}"
+}
+```
+
+**API Response Format**:
+```json
+{
+  "event": "message",
+  "answer": "你好世界",
+  "conversation_id": "xxx",
+  "metadata": {
+    "usage": {
+      "total_tokens": 54,
+      "latency": 1.26
+    }
+  }
+}
+```
+
+**Rationale**:
+- Simpler implementation than streaming
+- Adequate for batch text translation
+- Complete response in single call
+
+### Decision 3: Translation Batch Format
+
+**Choice**: Single text per request with translation prompt
+
+**Request Format**:
+```
+Translate the following text to {target_language}.
+Return ONLY the translated text, no explanations.
+
+{text_content}
+```
+
+**Rationale**:
+- Clear instruction for AI
+- Predictable response format
+- Easy to parse result
+
+### Decision 4: Translation Result Storage
+
+**Choice**: Independent JSON file per language (unchanged from previous design)
+
+```
+backend/storage/results/{task_id}/
+├── xxx_result.json              # Original
+├── xxx_translated_en.json       # English translation
+├── xxx_translated_ja.json       # Japanese translation
+└── ...
+```
+
+**Rationale**:
+- Non-destructive (original preserved)
+- Multiple languages supported
+- Easy to manage and delete
+- Clear file naming convention
+
+### Decision 5: Element Type Handling
+
+**Translatable types** (content is string):
+- `text`, `title`, `header`, `footer`, `paragraph`, `footnote`
+
+**Special handling** (content is dict):
+- `table` -> Translate `cells[].content`
+
+**Skip** (non-text content):
+- `page_number`, `image`, `chart`, `logo`, `reference`
+
+## Architecture
+
+### Component Diagram
+
+```
+┌─────────────────────────────────────────────────────────────┐
+│                        Frontend                              │
+│  ┌─────────────┐  ┌──────────────┐  ┌─────────────────────┐ │
+│  │ TaskDetail  │  │ TranslateBtn │  │ ProgressDisplay     │ │
+│  └─────────────┘  └──────────────┘  └─────────────────────┘ │
+└────────────────────────────┬────────────────────────────────┘
+                             │ HTTP
+┌────────────────────────────▼────────────────────────────────┐
+│                     Backend API                              │
+│  ┌─────────────────────────────────────────────────────────┐│
+│  │ TranslateRouter                                          ││
+│  │ POST /api/v2/translate/{task_id}                         ││
+│  │ GET  /api/v2/translate/{task_id}/status                  ││
+│  │ GET  /api/v2/translate/{task_id}/result                  ││
+│  └─────────────────────────────────────────────────────────┘│
+└────────────────────────────┬────────────────────────────────┘
+                             │
+┌────────────────────────────▼────────────────────────────────┐
+│                  TranslationService                          │
+│  ┌───────────────┐  ┌───────────────┐  ┌─────────────────┐  │
+│  │ DifyClient    │  │ BatchBuilder  │  │ ResultParser    │  │
+│  │ - translate() │  │ - extract()   │  │ - parse()       │  │
+│  │ - chat()      │  │ - format()    │  │ - map_ids()     │  │
+│  └───────────────┘  └───────────────┘  └─────────────────┘  │
+└────────────────────────────┬────────────────────────────────┘
+                             │ HTTPS
+┌────────────────────────────▼────────────────────────────────┐
+│                    DIFY AI Service                           │
+│                 https://dify.theaken.com/v1                  │
+│                    (Chat - Blocking)                         │
+└─────────────────────────────────────────────────────────────┘
+```
+
+### Translation JSON Schema
+
+```json
+{
+  "schema_version": "1.0.0",
+  "source_document": "xxx_result.json",
+  "source_lang": "auto",
+  "target_lang": "en",
+  "provider": "dify",
+  "translated_at": "2025-12-02T12:00:00Z",
+  "statistics": {
+    "total_elements": 50,
+    "translated_elements": 45,
+    "skipped_elements": 5,
+    "total_characters": 5000,
+    "processing_time_seconds": 30.5,
+    "total_tokens": 2500
+  },
+  "translations": {
+    "pp3_0_0": "Company Profile",
+    "pp3_0_1": "Founded in 2020...",
+    "table_1_0": {
+      "cells": [
+        {"row": 0, "col": 0, "content": "Technology"},
+        {"row": 0, "col": 1, "content": "Epoxy"}
+      ]
+    }
+  }
+}
+```
+
+### Language Code Mapping
+
+```python
+LANGUAGE_NAMES = {
+    "en": "English",
+    "zh-TW": "Traditional Chinese",
+    "zh-CN": "Simplified Chinese",
+    "ja": "Japanese",
+    "ko": "Korean",
+    "de": "German",
+    "fr": "French",
+    "es": "Spanish",
+    "pt": "Portuguese",
+    "it": "Italian",
+    "ru": "Russian",
+    "vi": "Vietnamese",
+    "th": "Thai",
+    # Additional languages as needed
+}
+```
+
+## Risks / Trade-offs
+
+### Risk 1: API Availability
+- **Risk**: DIFY service downtime affects translation
+- **Mitigation**: Add timeout handling, retry logic, graceful error messages
+
+### Risk 2: API Cost
+- **Risk**: High volume translation increases cost
+- **Mitigation**: Monitor usage via metadata, consider rate limiting
+
+### Risk 3: Network Latency
+- **Risk**: Each translation request adds network latency
+- **Mitigation**: Batch text when possible, show progress to user
+
+### Risk 4: Translation Quality Variance
+- **Risk**: AI translation quality varies by language pair
+- **Mitigation**: Document known limitations, allow user feedback
+
+## Migration Plan
+
+### Phase 1: Core Translation (This Proposal)
+1. DIFY client implementation
+2. Backend translation service (rewrite)
+3. API endpoints (modify)
+4. Frontend activation
+
+### Phase 2: Enhanced Features (Future)
+1. Translated PDF generation
+2. Translation caching
+3. Custom terminology support
+
+### Rollback
+- Translation is additive feature
+- No schema changes to existing data
+- Can disable by removing router registration
+
+## Open Questions
+
+1. **Rate Limiting**: Should we limit requests per minute to DIFY API?
+   - Tentative: 10 requests per minute per user
+
+2. **Retry Logic**: How to handle API failures?
+   - Tentative: Retry up to 3 times with exponential backoff
+
+3. **Batch Size**: How many elements per API call?
+   - Tentative: 1 element per call for simplicity, optimize later if needed
diff --git a/openspec/changes/archive/2025-12-02-add-document-translation/proposal.md b/openspec/changes/archive/2025-12-02-add-document-translation/proposal.md
new file mode 100644
index 0000000..1fb7f91
--- /dev/null
+++ b/openspec/changes/archive/2025-12-02-add-document-translation/proposal.md
@@ -0,0 +1,54 @@
+# Change: Add Document Translation Feature
+
+## Why
+
+Users need to translate OCR-processed documents into different languages while preserving the original layout. Currently, the system only extracts text but cannot translate it. This feature enables multilingual document processing using DIFY AI service, providing high-quality translations with simple API integration.
+
+## What Changes
+
+- **NEW**: Translation service using DIFY AI API (Chat mode, Blocking)
+- **NEW**: Translation REST API endpoints (`/api/v2/translate/*`)
+- **NEW**: Translation result JSON format (independent file per target language)
+- **UPDATE**: Frontend translation UI activation with progress display
+- **REMOVED**: Local MADLAD-400-3B model (replaced with DIFY API)
+- **REMOVED**: GPU memory management for translation (no longer needed)
+
+## Impact
+
+- Affected specs:
+  - NEW `specs/translation/spec.md` - Core translation capability
+  - MODIFY `specs/result-export/spec.md` - Add translation JSON export format
+
+- Affected code:
+  - `backend/app/services/translation_service.py` (REWRITE - use DIFY API)
+  - `backend/app/routers/translate.py` (MODIFY)
+  - `backend/app/schemas/translation.py` (MODIFY)
+  - `frontend/src/pages/TaskDetailPage.tsx` (MODIFY)
+  - `frontend/src/services/api.ts` (MODIFY)
+
+## Technical Summary
+
+### Translation Service
+- Provider: DIFY AI (theaken.com)
+- Mode: Chat (Blocking response)
+- Base URL: `https://dify.theaken.com/v1`
+- Endpoint: `POST /chat-messages`
+- API Key: `app-YOPrF2ro5fshzMkCZviIuUJd`
+
+### Benefits over Local Model
+| Aspect | DIFY API | Local MADLAD-400 |
+|--------|----------|------------------|
+| Quality | High (cloud AI) | Variable |
+| Setup | No model download | 12GB download |
+| GPU Usage | None | 2-3GB VRAM |
+| Latency | ~1-2s per request | Fast after load |
+| Maintenance | API provider managed | Self-managed |
+
+### Data Flow
+1. Read `xxx_result.json` (UnifiedDocument format)
+2. Extract translatable elements (text, title, header, footer, paragraph, footnote, table cells)
+3. Send to DIFY API with translation prompt
+4. Parse response and save to `xxx_translated_{lang}.json`
+
+### Unified Processing
+All three tracks (Direct/OCR/Hybrid) use the same UnifiedDocument format, enabling unified translation logic without track-specific handling.
diff --git a/openspec/changes/archive/2025-12-02-add-document-translation/specs/result-export/spec.md b/openspec/changes/archive/2025-12-02-add-document-translation/specs/result-export/spec.md
new file mode 100644
index 0000000..3b8ac65
--- /dev/null
+++ b/openspec/changes/archive/2025-12-02-add-document-translation/specs/result-export/spec.md
@@ -0,0 +1,55 @@
+## ADDED Requirements
+
+### Requirement: Translation Result JSON Export
+
+The system SHALL support exporting translation results as independent JSON files following a defined schema.
+
+#### Scenario: Export translation result JSON
+- **WHEN** translation completes for a document
+- **THEN** system SHALL save translation to `{filename}_translated_{lang}.json`
+- **AND** file SHALL be stored alongside original `{filename}_result.json`
+- **AND** original result file SHALL remain unchanged
+
+#### Scenario: Translation JSON schema compliance
+- **WHEN** translation result is saved
+- **THEN** JSON SHALL include schema_version field ("1.0.0")
+- **AND** SHALL include source_document reference
+- **AND** SHALL include source_lang and target_lang
+- **AND** SHALL include provider identifier (e.g., "dify")
+- **AND** SHALL include translated_at timestamp
+- **AND** SHALL include translations dict mapping element_id to translated content
+
+#### Scenario: Translation statistics in export
+- **WHEN** translation result is saved
+- **THEN** JSON SHALL include statistics object with:
+  - total_elements: count of all elements in document
+  - translated_elements: count of successfully translated elements
+  - skipped_elements: count of non-translatable elements (images, charts, etc.)
+  - total_characters: character count of translated text
+  - processing_time_seconds: translation duration
+
+#### Scenario: Table cell translation in export
+- **WHEN** document contains tables
+- **THEN** translation JSON SHALL represent table translations as:
+  ```json
+  {
+    "table_1_0": {
+      "cells": [
+        {"row": 0, "col": 0, "content": "Translated cell text"},
+        {"row": 0, "col": 1, "content": "Another cell"}
+      ]
+    }
+  }
+  ```
+- **AND** row/col positions SHALL match original table structure
+
+#### Scenario: Download translation result via API
+- **WHEN** GET request to `/api/v2/translate/{task_id}/result?lang={lang}`
+- **THEN** system SHALL return translation JSON content
+- **AND** Content-Type SHALL be application/json
+- **AND** response SHALL include appropriate cache headers
+
+#### Scenario: List available translations
+- **WHEN** GET request to `/api/v2/tasks/{task_id}/translations`
+- **THEN** system SHALL return list of available translation languages
+- **AND** include translation metadata (translated_at, provider, statistics)
diff --git a/openspec/changes/archive/2025-12-02-add-document-translation/specs/translation/spec.md b/openspec/changes/archive/2025-12-02-add-document-translation/specs/translation/spec.md
new file mode 100644
index 0000000..3f2b8aa
--- /dev/null
+++ b/openspec/changes/archive/2025-12-02-add-document-translation/specs/translation/spec.md
@@ -0,0 +1,184 @@
+## ADDED Requirements
+
+### Requirement: Document Translation Service
+
+The system SHALL provide a document translation service that translates extracted text from OCR-processed documents into target languages using DIFY AI API.
+
+#### Scenario: Successful translation of Direct track document
+- **GIVEN** a completed OCR task with Direct track processing
+- **WHEN** user requests translation to English
+- **THEN** the system extracts all translatable elements (text, title, header, footer, paragraph, footnote, table cells)
+- **AND** translates them using DIFY AI API
+- **AND** saves the result to `{task_id}_translated_en.json`
+
+#### Scenario: Successful translation of OCR track document
+- **GIVEN** a completed OCR task with OCR track processing
+- **WHEN** user requests translation to Japanese
+- **THEN** the system extracts all translatable elements from UnifiedDocument format
+- **AND** translates them preserving element_id mapping
+- **AND** saves the result to `{task_id}_translated_ja.json`
+
+#### Scenario: Successful translation of Hybrid track document
+- **GIVEN** a completed OCR task with Hybrid track processing
+- **WHEN** translation is requested
+- **THEN** the system processes the document using the same unified logic
+- **AND** handles any combination of element types present
+
+#### Scenario: Table cell translation
+- **GIVEN** a document containing table elements
+- **WHEN** translation is requested
+- **THEN** the system extracts text from each table cell
+- **AND** translates each cell content individually
+- **AND** preserves row/col position in the translation result
+
+---
+
+### Requirement: Translation API Endpoints
+
+The system SHALL expose REST API endpoints for translation operations.
+
+#### Scenario: Start translation request
+- **GIVEN** a completed OCR task with task_id
+- **WHEN** POST request to `/api/v2/translate/{task_id}` with target_lang parameter
+- **THEN** the system starts background translation process
+- **AND** returns translation job status with 202 Accepted
+
+#### Scenario: Query translation status
+- **GIVEN** an active translation job
+- **WHEN** GET request to `/api/v2/translate/{task_id}/status`
+- **THEN** the system returns current status (pending, translating, completed, failed)
+- **AND** includes progress information (current_element, total_elements)
+
+#### Scenario: Retrieve translation result
+- **GIVEN** a completed translation job
+- **WHEN** GET request to `/api/v2/translate/{task_id}/result?lang={target_lang}`
+- **THEN** the system returns the translation JSON content
+
+#### Scenario: Translation for non-existent task
+- **GIVEN** an invalid or non-existent task_id
+- **WHEN** translation is requested
+- **THEN** the system returns 404 Not Found error
+
+---
+
+### Requirement: DIFY API Integration
+
+The system SHALL integrate with DIFY AI service for translation.
+
+#### Scenario: API request format
+- **GIVEN** text to be translated
+- **WHEN** calling DIFY API
+- **THEN** the system sends POST request to `/chat-messages` endpoint
+- **AND** includes query with translation prompt
+- **AND** uses blocking response mode
+- **AND** includes user identifier for tracking
+
+#### Scenario: API response handling
+- **GIVEN** DIFY API returns translation response
+- **WHEN** parsing the response
+- **THEN** the system extracts translated text from `answer` field
+- **AND** records usage statistics (tokens, latency)
+
+#### Scenario: API error handling
+- **GIVEN** DIFY API returns error or times out
+- **WHEN** handling the error
+- **THEN** the system retries up to 3 times with exponential backoff
+- **AND** returns appropriate error message if all retries fail
+
+#### Scenario: API rate limiting
+- **GIVEN** high volume of translation requests
+- **WHEN** requests approach rate limits
+- **THEN** the system queues requests appropriately
+- **AND** provides feedback about wait times
+
+---
+
+### Requirement: Translation Prompt Format
+
+The system SHALL use structured prompts for translation requests.
+
+#### Scenario: Generate translation prompt
+- **GIVEN** source text to translate
+- **WHEN** preparing DIFY API request
+- **THEN** the system formats prompt as:
+  ```
+  Translate the following text to {language}.
+  Return ONLY the translated text, no explanations.
+
+  {text}
+  ```
+
+#### Scenario: Language name mapping
+- **GIVEN** language code like "zh-TW" or "ja"
+- **WHEN** constructing translation prompt
+- **THEN** the system maps to full language name (Traditional Chinese, Japanese)
+
+---
+
+### Requirement: Translation Progress Reporting
+
+The system SHALL provide real-time progress feedback during translation.
+
+#### Scenario: Progress during multi-element translation
+- **GIVEN** a document with 50 translatable elements
+- **WHEN** user queries status
+- **THEN** the system returns progress like `{"status": "translating", "current_element": 25, "total_elements": 50}`
+
+#### Scenario: Translation starting status
+- **GIVEN** translation job just started
+- **WHEN** user queries status
+- **THEN** the system returns `{"status": "pending"}`
+
+---
+
+### Requirement: Translation Result Storage
+
+The system SHALL store translation results as independent JSON files.
+
+#### Scenario: Save translation result
+- **GIVEN** translation completes successfully
+- **WHEN** saving results
+- **THEN** the system creates `{original_filename}_translated_{lang}.json`
+- **AND** includes schema_version, metadata, and translations dict
+
+#### Scenario: Multiple language translations
+- **GIVEN** a document translated to English and Japanese
+- **WHEN** checking result files
+- **THEN** both `xxx_translated_en.json` and `xxx_translated_ja.json` exist
+- **AND** original `xxx_result.json` is unchanged
+
+---
+
+### Requirement: Language Support
+
+The system SHALL support common languages through DIFY AI service.
+
+#### Scenario: Common language translation
+- **GIVEN** target language is English, Chinese, Japanese, or Korean
+- **WHEN** translation is requested
+- **THEN** the system includes appropriate language name in prompt
+- **AND** executes translation successfully
+
+#### Scenario: Automatic source language detection
+- **GIVEN** source_lang is set to "auto"
+- **WHEN** translation is executed
+- **THEN** the AI model automatically detects source language
+- **AND** translates to target language
+
+#### Scenario: Supported languages list
+- **GIVEN** user queries supported languages
+- **WHEN** checking language support
+- **THEN** the system provides list including:
+  - English (en)
+  - Traditional Chinese (zh-TW)
+  - Simplified Chinese (zh-CN)
+  - Japanese (ja)
+  - Korean (ko)
+  - German (de)
+  - French (fr)
+  - Spanish (es)
+  - Portuguese (pt)
+  - Italian (it)
+  - Russian (ru)
+  - Vietnamese (vi)
+  - Thai (th)
diff --git a/openspec/changes/archive/2025-12-02-add-document-translation/tasks.md b/openspec/changes/archive/2025-12-02-add-document-translation/tasks.md
new file mode 100644
index 0000000..db4fd3f
--- /dev/null
+++ b/openspec/changes/archive/2025-12-02-add-document-translation/tasks.md
@@ -0,0 +1,121 @@
+# Implementation Tasks
+
+## 1. Backend - DIFY Client
+
+- [x] 1.1 Create DIFY client (`backend/app/services/dify_client.py`)
+  - HTTP client with httpx
+  - Base URL: `https://dify.theaken.com/v1`
+  - API Key configuration
+  - `translate(text, target_lang)` and `translate_batch(texts, target_lang)` methods
+  - Error handling and retry logic (3 retries, exponential backoff)
+
+- [x] 1.2 Add translation prompt template
+  - Format: "Translate the following text to {language}. Return ONLY the translated text, no explanations.\n\n{text}"
+  - Batch format with numbered markers [1], [2], [3]...
+  - Language name mapping (en → English, zh-TW → Traditional Chinese, etc.)
+
+## 2. Backend - Translation Service
+
+- [x] 2.1 Rewrite translation service (`backend/app/services/translation_service.py`)
+  - Use DIFY client instead of local model
+  - Element extraction from UnifiedDocument (all track types)
+  - Batch translation (MAX_BATCH_CHARS=5000, MAX_BATCH_ITEMS=20)
+  - Result parsing and element_id mapping
+
+- [x] 2.2 Create translation result JSON writer
+  - Schema version, metadata, translations dict
+  - Table cell handling with row/col positions
+  - Save to `{task_id}_translated_{lang}.json`
+  - Include usage statistics (tokens, latency, batch_count)
+
+- [x] 2.3 Add translatable element type handling
+  - Text types: `text`, `title`, `header`, `footer`, `paragraph`, `footnote`
+  - Table: Extract and translate `cells[].content`
+  - Skip: `page_number`, `image`, `chart`, `logo`, `reference`
+
+## 3. Backend - API Endpoints
+
+- [x] 3.1 Create/Update translation router (`backend/app/routers/translate.py`)
+  - POST `/api/v2/translate/{task_id}` - Start translation
+  - GET `/api/v2/translate/{task_id}/status` - Get progress
+  - GET `/api/v2/translate/{task_id}/result` - Get translation result
+  - GET `/api/v2/translate/{task_id}/translations` - List available translations
+  - DELETE `/api/v2/translate/{task_id}/translations/{lang}` - Delete translation
+
+- [x] 3.2 Implement background task processing
+  - Use FastAPI BackgroundTasks for async translation
+  - Status tracking (pending, translating, completed, failed)
+  - Progress reporting (current element / total elements)
+
+- [x] 3.3 Add translation schemas (`backend/app/schemas/translation.py`)
+  - TranslationRequest (task_id, target_lang)
+  - TranslationStatusResponse (status, progress, error)
+  - TranslationListResponse (translations, statistics)
+
+- [x] 3.4 Register router in main app
+
+## 4. Frontend - UI Updates
+
+- [x] 4.1 Enable translation UI in TaskDetailPage
+  - Translation state management
+  - Language selector connected to state
+
+- [x] 4.2 Add translation progress display
+  - Progress tracking
+  - Status polling (translating element X/Y)
+  - Error handling and display
+
+- [x] 4.3 Update API service
+  - Implement startTranslation method
+  - Add polling for translation status
+  - Handle translation result
+
+- [x] 4.4 Add translation complete state
+  - Show success message
+  - Display available translated versions
+
+## 5. Testing
+
+Use existing JSON files in `backend/storage/results/` for testing.
+
+Available test samples:
+- Direct track: `1c94bfbf-*/edit_result.json`, `8eedd9ed-*/ppt_result.json`
+- OCR track: `c85fff69-*/scan_result.json`, `ca2b59a3-*/img3_result.json`
+- Hybrid track: `1484ba43-*/edit2_result.json`
+
+- [x] 5.1 Unit tests for DIFY client
+  - Test with real API calls (no mocks)
+  - Test retry logic on timeout
+
+- [x] 5.2 Unit tests for translation service
+  - Element extraction from existing result.json files (10 tests pass)
+  - Result parsing and element_id mapping
+  - Table cell extraction and translation
+
+- [x] 5.3 Integration tests for API endpoints
+  - Start translation with existing task_id
+  - Status polling during translation
+  - Result retrieval after completion
+
+- [x] 5.4 Manual E2E verification
+  - Translate Direct track document (edit_result.json → zh-TW) ✓
+  - Verified translation quality and JSON structure
+
+## 6. Configuration
+
+- [x] 6.1 Add DIFY configuration (hardcoded in dify_client.py)
+  - `DIFY_BASE_URL`: https://dify.theaken.com/v1
+  - `DIFY_API_KEY`: app-YOPrF2ro5fshzMkCZviIuUJd
+  - `DIFY_TIMEOUT`: 120 seconds
+  - `DIFY_MAX_RETRIES`: 3
+  - `MAX_BATCH_CHARS`: 5000
+  - `MAX_BATCH_ITEMS`: 20
+
+## 7. Documentation
+
+- [ ] 7.1 Update API documentation
+  - Add translation endpoints to OpenAPI spec
+
+- [ ] 7.2 Add DIFY setup instructions
+  - API key configuration
+  - Rate limiting considerations
diff --git a/openspec/specs/result-export/spec.md b/openspec/specs/result-export/spec.md
index 4c7e6ad..9a74912 100644
--- a/openspec/specs/result-export/spec.md
+++ b/openspec/specs/result-export/spec.md
@@ -114,3 +114,57 @@ The system SHALL provide export formats that preserve document structure for dow
 - **AND** identify layout regions (header, body, footer, sidebar)
 - **AND** provide confidence scores for layout detection
 
+### Requirement: Translation Result JSON Export
+
+The system SHALL support exporting translation results as independent JSON files following a defined schema.
+
+#### Scenario: Export translation result JSON
+- **WHEN** translation completes for a document
+- **THEN** system SHALL save translation to `{filename}_translated_{lang}.json`
+- **AND** file SHALL be stored alongside original `{filename}_result.json`
+- **AND** original result file SHALL remain unchanged
+
+#### Scenario: Translation JSON schema compliance
+- **WHEN** translation result is saved
+- **THEN** JSON SHALL include schema_version field ("1.0.0")
+- **AND** SHALL include source_document reference
+- **AND** SHALL include source_lang and target_lang
+- **AND** SHALL include provider identifier (e.g., "dify")
+- **AND** SHALL include translated_at timestamp
+- **AND** SHALL include translations dict mapping element_id to translated content
+
+#### Scenario: Translation statistics in export
+- **WHEN** translation result is saved
+- **THEN** JSON SHALL include statistics object with:
+  - total_elements: count of all elements in document
+  - translated_elements: count of successfully translated elements
+  - skipped_elements: count of non-translatable elements (images, charts, etc.)
+  - total_characters: character count of translated text
+  - processing_time_seconds: translation duration
+
+#### Scenario: Table cell translation in export
+- **WHEN** document contains tables
+- **THEN** translation JSON SHALL represent table translations as:
+  ```json
+  {
+    "table_1_0": {
+      "cells": [
+        {"row": 0, "col": 0, "content": "Translated cell text"},
+        {"row": 0, "col": 1, "content": "Another cell"}
+      ]
+    }
+  }
+  ```
+- **AND** row/col positions SHALL match original table structure
+
+#### Scenario: Download translation result via API
+- **WHEN** GET request to `/api/v2/translate/{task_id}/result?lang={lang}`
+- **THEN** system SHALL return translation JSON content
+- **AND** Content-Type SHALL be application/json
+- **AND** response SHALL include appropriate cache headers
+
+#### Scenario: List available translations
+- **WHEN** GET request to `/api/v2/tasks/{task_id}/translations`
+- **THEN** system SHALL return list of available translation languages
+- **AND** include translation metadata (translated_at, provider, statistics)
+
diff --git a/openspec/specs/translation/spec.md b/openspec/specs/translation/spec.md
new file mode 100644
index 0000000..ff30e78
--- /dev/null
+++ b/openspec/specs/translation/spec.md
@@ -0,0 +1,188 @@
+# translation Specification
+
+## Purpose
+TBD - created by archiving change add-document-translation. Update Purpose after archive.
+## Requirements
+### Requirement: Document Translation Service
+
+The system SHALL provide a document translation service that translates extracted text from OCR-processed documents into target languages using DIFY AI API.
+
+#### Scenario: Successful translation of Direct track document
+- **GIVEN** a completed OCR task with Direct track processing
+- **WHEN** user requests translation to English
+- **THEN** the system extracts all translatable elements (text, title, header, footer, paragraph, footnote, table cells)
+- **AND** translates them using DIFY AI API
+- **AND** saves the result to `{task_id}_translated_en.json`
+
+#### Scenario: Successful translation of OCR track document
+- **GIVEN** a completed OCR task with OCR track processing
+- **WHEN** user requests translation to Japanese
+- **THEN** the system extracts all translatable elements from UnifiedDocument format
+- **AND** translates them preserving element_id mapping
+- **AND** saves the result to `{task_id}_translated_ja.json`
+
+#### Scenario: Successful translation of Hybrid track document
+- **GIVEN** a completed OCR task with Hybrid track processing
+- **WHEN** translation is requested
+- **THEN** the system processes the document using the same unified logic
+- **AND** handles any combination of element types present
+
+#### Scenario: Table cell translation
+- **GIVEN** a document containing table elements
+- **WHEN** translation is requested
+- **THEN** the system extracts text from each table cell
+- **AND** translates each cell content individually
+- **AND** preserves row/col position in the translation result
+
+---
+
+### Requirement: Translation API Endpoints
+
+The system SHALL expose REST API endpoints for translation operations.
+
+#### Scenario: Start translation request
+- **GIVEN** a completed OCR task with task_id
+- **WHEN** POST request to `/api/v2/translate/{task_id}` with target_lang parameter
+- **THEN** the system starts background translation process
+- **AND** returns translation job status with 202 Accepted
+
+#### Scenario: Query translation status
+- **GIVEN** an active translation job
+- **WHEN** GET request to `/api/v2/translate/{task_id}/status`
+- **THEN** the system returns current status (pending, translating, completed, failed)
+- **AND** includes progress information (current_element, total_elements)
+
+#### Scenario: Retrieve translation result
+- **GIVEN** a completed translation job
+- **WHEN** GET request to `/api/v2/translate/{task_id}/result?lang={target_lang}`
+- **THEN** the system returns the translation JSON content
+
+#### Scenario: Translation for non-existent task
+- **GIVEN** an invalid or non-existent task_id
+- **WHEN** translation is requested
+- **THEN** the system returns 404 Not Found error
+
+---
+
+### Requirement: DIFY API Integration
+
+The system SHALL integrate with DIFY AI service for translation.
+
+#### Scenario: API request format
+- **GIVEN** text to be translated
+- **WHEN** calling DIFY API
+- **THEN** the system sends POST request to `/chat-messages` endpoint
+- **AND** includes query with translation prompt
+- **AND** uses blocking response mode
+- **AND** includes user identifier for tracking
+
+#### Scenario: API response handling
+- **GIVEN** DIFY API returns translation response
+- **WHEN** parsing the response
+- **THEN** the system extracts translated text from `answer` field
+- **AND** records usage statistics (tokens, latency)
+
+#### Scenario: API error handling
+- **GIVEN** DIFY API returns error or times out
+- **WHEN** handling the error
+- **THEN** the system retries up to 3 times with exponential backoff
+- **AND** returns appropriate error message if all retries fail
+
+#### Scenario: API rate limiting
+- **GIVEN** high volume of translation requests
+- **WHEN** requests approach rate limits
+- **THEN** the system queues requests appropriately
+- **AND** provides feedback about wait times
+
+---
+
+### Requirement: Translation Prompt Format
+
+The system SHALL use structured prompts for translation requests.
+
+#### Scenario: Generate translation prompt
+- **GIVEN** source text to translate
+- **WHEN** preparing DIFY API request
+- **THEN** the system formats prompt as:
+  ```
+  Translate the following text to {language}.
+  Return ONLY the translated text, no explanations.
+
+  {text}
+  ```
+
+#### Scenario: Language name mapping
+- **GIVEN** language code like "zh-TW" or "ja"
+- **WHEN** constructing translation prompt
+- **THEN** the system maps to full language name (Traditional Chinese, Japanese)
+
+---
+
+### Requirement: Translation Progress Reporting
+
+The system SHALL provide real-time progress feedback during translation.
+
+#### Scenario: Progress during multi-element translation
+- **GIVEN** a document with 50 translatable elements
+- **WHEN** user queries status
+- **THEN** the system returns progress like `{"status": "translating", "current_element": 25, "total_elements": 50}`
+
+#### Scenario: Translation starting status
+- **GIVEN** translation job just started
+- **WHEN** user queries status
+- **THEN** the system returns `{"status": "pending"}`
+
+---
+
+### Requirement: Translation Result Storage
+
+The system SHALL store translation results as independent JSON files.
+
+#### Scenario: Save translation result
+- **GIVEN** translation completes successfully
+- **WHEN** saving results
+- **THEN** the system creates `{original_filename}_translated_{lang}.json`
+- **AND** includes schema_version, metadata, and translations dict
+
+#### Scenario: Multiple language translations
+- **GIVEN** a document translated to English and Japanese
+- **WHEN** checking result files
+- **THEN** both `xxx_translated_en.json` and `xxx_translated_ja.json` exist
+- **AND** original `xxx_result.json` is unchanged
+
+---
+
+### Requirement: Language Support
+
+The system SHALL support common languages through DIFY AI service.
+
+#### Scenario: Common language translation
+- **GIVEN** target language is English, Chinese, Japanese, or Korean
+- **WHEN** translation is requested
+- **THEN** the system includes appropriate language name in prompt
+- **AND** executes translation successfully
+
+#### Scenario: Automatic source language detection
+- **GIVEN** source_lang is set to "auto"
+- **WHEN** translation is executed
+- **THEN** the AI model automatically detects source language
+- **AND** translates to target language
+
+#### Scenario: Supported languages list
+- **GIVEN** user queries supported languages
+- **WHEN** checking language support
+- **THEN** the system provides list including:
+  - English (en)
+  - Traditional Chinese (zh-TW)
+  - Simplified Chinese (zh-CN)
+  - Japanese (ja)
+  - Korean (ko)
+  - German (de)
+  - French (fr)
+  - Spanish (es)
+  - Portuguese (pt)
+  - Italian (it)
+  - Russian (ru)
+  - Vietnamese (vi)
+  - Thai (th)
+