feat: add multilingual font support for translated PDFs

- Add NotoSansKR and NotoSansThai fonts for Korean and Thai language support
- Update download_fonts.sh to download all required fonts
- Add LANGUAGE_FONT_MAP for language-to-font mapping in pdf_generator_service.py
- Add get_font_for_language() method to select appropriate font based on target language
- Update _get_reflow_styles() to accept target_lang parameter
- Pass target_lang through generate_translated_pdf() to PDF generation methods
- Fix garbled characters (亂碼) issue for Korean and Thai translations

Supported languages:
- Chinese (zh-CN/zh-TW), Japanese (ja): NotoSansSC
- Korean (ko): NotoSansKR
- Thai (th): NotoSansThai
- Russian, Vietnamese, Latin languages: NotoSansSC

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/services/pdf_generator_service.py

@@ -145,40 +145,83 @@ class PDFGeneratorService:
     STYLE_FLAG_UNDERLINE = 4
     STYLE_FLAG_STRIKETHROUGH = 8
 
+    # Language to font mapping for proper character support
+    LANGUAGE_FONT_MAP = {
+        # Korean requires separate font
+        'ko': 'NotoSansKR',
+        # Thai requires separate font
+        'th': 'NotoSansThai',
+        # All other languages use NotoSansSC (covers CJK, Latin, Cyrillic, etc.)
+        'default': 'NotoSansSC',
+    }
+
     def __init__(self):
         """Initialize PDF generator with font configuration"""
-        self.font_name = 'NotoSansSC'
+        self.font_name = 'NotoSansSC'  # Default font
         self.font_path = None
         self.font_registered = False
         self.current_processing_track = None  # Track type for current document
+        self.registered_fonts = {}  # Track which fonts are registered
 
-        self._register_chinese_font()
+        self._register_multilingual_fonts()
 
-    def _register_chinese_font(self):
+    def _register_multilingual_fonts(self):
-        """Register Chinese font for PDF generation"""
+        """Register fonts for all supported languages"""
-        try:
+        project_root = Path(__file__).resolve().parent.parent.parent.parent
-            # Get font path from settings
+        fonts_dir = project_root / 'backend' / 'fonts'
-            font_path = Path(settings.chinese_font_path)
 
-            # Try relative path from project root
+        # Font files to register
-            if not font_path.is_absolute():
+        font_files = {
-                # Adjust path - settings.chinese_font_path starts with ./backend/
+            'NotoSansSC': 'NotoSansSC-Regular.ttf',
-                project_root = Path(__file__).resolve().parent.parent.parent.parent
+            'NotoSansKR': 'NotoSansKR-Regular.ttf',
-                font_path = project_root / font_path
+            'NotoSansThai': 'NotoSansThai-Regular.ttf',
+        }
 
-            if not font_path.exists():
+        for font_name, font_file in font_files.items():
-                logger.error(f"Chinese font not found at {font_path}")
+            font_path = fonts_dir / font_file
-                return
+            if font_path.exists():
+                try:
+                    pdfmetrics.registerFont(TTFont(font_name, str(font_path)))
+                    self.registered_fonts[font_name] = font_path
+                    logger.info(f"Font registered: {font_name} from {font_path}")
+                except Exception as e:
+                    logger.warning(f"Failed to register font {font_name}: {e}")
+            else:
+                logger.warning(f"Font file not found: {font_path}")
 
-            # Register font
+        # Set default font
-            pdfmetrics.registerFont(TTFont(self.font_name, str(font_path)))
+        if 'NotoSansSC' in self.registered_fonts:
-            self.font_path = font_path
+            self.font_name = 'NotoSansSC'
+            self.font_path = self.registered_fonts['NotoSansSC']
             self.font_registered = True
-            logger.info(f"Chinese font registered: {self.font_name} from {font_path}")
+        else:
-
-        except Exception as e:
-            logger.error(f"Failed to register Chinese font: {e}")
             self.font_registered = False
+            logger.error("Default font NotoSansSC not available")
+
+    def get_font_for_language(self, target_lang: str) -> str:
+        """
+        Get the appropriate font name for a target language.
+
+        Args:
+            target_lang: Language code (e.g., 'ko', 'th', 'ja', 'zh-TW')
+
+        Returns:
+            Font name to use for this language
+        """
+        # Check if language has a specific font
+        font_name = self.LANGUAGE_FONT_MAP.get(target_lang, self.LANGUAGE_FONT_MAP['default'])
+
+        # Verify font is registered, fall back to default if not
+        if font_name not in self.registered_fonts:
+            logger.warning(f"Font {font_name} for language {target_lang} not registered, using default")
+            font_name = self.LANGUAGE_FONT_MAP['default']
+
+        # Final fallback to Helvetica if no fonts available
+        if font_name not in self.registered_fonts:
+            logger.warning(f"No suitable font found for language {target_lang}, using Helvetica")
+            return 'Helvetica'
+
+        return font_name
 
     def _detect_content_orientation(
         self,
@@ -4462,10 +4505,20 @@ class PDFGeneratorService:
             # Direct track: elements already in reading order from PyMuPDF
             return elements
 
-    def _get_reflow_styles(self) -> Dict[str, ParagraphStyle]:
+    def _get_reflow_styles(self, target_lang: Optional[str] = None) -> Dict[str, ParagraphStyle]:
-        """Create consistent styles for reflow PDF generation."""
+        """
+        Create consistent styles for reflow PDF generation.
+
+        Args:
+            target_lang: Optional target language code for selecting appropriate font.
+                         If not provided, uses default font.
+        """
         base_styles = getSampleStyleSheet()
-        font_name = self.font_name if self.font_registered else 'Helvetica'
+        # Use language-specific font if target_lang is provided
+        if target_lang:
+            font_name = self.get_font_for_language(target_lang)
+        else:
+            font_name = self.font_name if self.font_registered else 'Helvetica'
 
         styles = {
             'Title': ParagraphStyle(
@@ -4936,7 +4989,8 @@ class PDFGeneratorService:
         json_path: Path,
         output_path: Path,
         source_file_path: Optional[Path] = None,
-        use_elements_only: bool = False
+        use_elements_only: bool = False,
+        target_lang: Optional[str] = None
     ) -> bool:
         """
         Generate reflow layout PDF from OCR/Direct JSON data.
@@ -4953,6 +5007,7 @@ class PDFGeneratorService:
             source_file_path: Optional path to original source file (for images)
             use_elements_only: If True, always use elements from JSON (for translated PDFs
                               where translations are applied to elements, not raw_ocr_regions)
+            target_lang: Optional target language code for selecting appropriate font
 
         Returns:
             True if successful, False otherwise
@@ -4969,8 +5024,8 @@ class PDFGeneratorService:
             is_ocr_track = processing_track == 'ocr'
             logger.info(f"Reflow PDF generation - Processing track: {processing_track}")
 
-            # Get styles
+            # Get styles (with language-specific font if target_lang provided)
-            styles = self._get_reflow_styles()
+            styles = self._get_reflow_styles(target_lang=target_lang)
 
             # Build document content
             story = []
@@ -5149,7 +5204,8 @@ class PDFGeneratorService:
                     result_json=result_json,
                     raw_ocr_translations=raw_ocr_translations,
                     output_path=output_path,
-                    result_dir=result_json_path.parent
+                    result_dir=result_json_path.parent,
+                    target_lang=target_lang
                 )
 
             # Direct Track: Use element-based translations
@@ -5189,7 +5245,8 @@ class PDFGeneratorService:
                     json_path=tmp_path,
                     output_path=output_path,
                     source_file_path=result_json_path.parent,  # Contains extracted images
-                    use_elements_only=True  # Use elements with translations applied
+                    use_elements_only=True,  # Use elements with translations applied
+                    target_lang=target_lang  # Use language-specific font
                 )
                 return success
             finally:
@@ -5214,7 +5271,8 @@ class PDFGeneratorService:
         result_json: Dict,
         raw_ocr_translations: List[Dict],
         output_path: Path,
-        result_dir: Path
+        result_dir: Path,
+        target_lang: Optional[str] = None
     ) -> bool:
         """
         Generate translated reflow PDF for OCR Track documents.
@@ -5226,13 +5284,14 @@ class PDFGeneratorService:
             raw_ocr_translations: List of {page, index, original, translated}
             output_path: Path to save generated PDF
             result_dir: Path to result directory for images
+            target_lang: Target language code for selecting appropriate font
 
         Returns:
             True if successful, False otherwise
         """
         try:
-            # Get styles
+            # Get styles with language-specific font
-            styles = self._get_reflow_styles()
+            styles = self._get_reflow_styles(target_lang=target_lang)
 
             # Build document content
             story = []

backend/download_fonts.sh

@@ -1,31 +1,56 @@
 #!/bin/bash
-# Download Noto Sans SC TrueType font for layout-preserving PDF generation
+# Download Noto Sans fonts for multilingual PDF generation
+# Supports: CJK (Chinese, Japanese, Korean), Thai, Vietnamese, Russian, etc.
 
 set -e
 
 FONT_DIR="backend/fonts"
-FONT_URL="https://github.com/notofonts/noto-cjk/raw/main/Sans/Variable/TTF/Subset/NotoSansSC-VF.ttf"
-FONT_FILE="NotoSansSC-Regular.ttf"
 
-echo "🔤 Downloading Chinese font for PDF generation..."
+echo "🔤 Downloading fonts for multilingual PDF generation..."
 
 # Create font directory
 mkdir -p "$FONT_DIR"
 
-# Download font if not exists
+# Function to download font
-if [ -f "$FONT_DIR/$FONT_FILE" ]; then
+download_font() {
-    echo "✓ Font already exists: $FONT_DIR/$FONT_FILE"
+    local url="$1"
-else
+    local file="$2"
-    echo "Downloading from GitHub..."
+    local desc="$3"
-    wget "$FONT_URL" -O "$FONT_DIR/$FONT_FILE"
 
-    if [ -f "$FONT_DIR/$FONT_FILE" ]; then
+    if [ -f "$FONT_DIR/$file" ]; then
-        SIZE=$(du -h "$FONT_DIR/$FONT_FILE" | cut -f1)
+        echo "✓ $desc already exists"
-        echo "✓ Font downloaded successfully: $SIZE"
     else
-        echo "✗ Font download failed"
+        echo "Downloading $desc..."
-        exit 1
+        if wget -q "$url" -O "$FONT_DIR/$file"; then
+            SIZE=$(du -h "$FONT_DIR/$file" | cut -f1)
+            echo "✓ $desc downloaded: $SIZE"
+        else
+            echo "✗ Failed to download $desc"
+            return 1
+        fi
     fi
-fi
+}
 
+# NotoSansSC - Chinese (Simplified), also covers Japanese and basic CJK
+download_font \
+    "https://github.com/notofonts/noto-cjk/raw/main/Sans/Variable/TTF/Subset/NotoSansSC-VF.ttf" \
+    "NotoSansSC-Regular.ttf" \
+    "Noto Sans SC (Chinese/Japanese)"
+
+# NotoSansKR - Korean
+download_font \
+    "https://github.com/notofonts/noto-cjk/raw/main/Sans/Variable/TTF/Subset/NotoSansKR-VF.ttf" \
+    "NotoSansKR-Regular.ttf" \
+    "Noto Sans KR (Korean)"
+
+# NotoSansThai - Thai
+download_font \
+    "https://github.com/notofonts/noto-fonts/raw/main/hinted/ttf/NotoSansThai/NotoSansThai-Regular.ttf" \
+    "NotoSansThai-Regular.ttf" \
+    "Noto Sans Thai"
+
+echo ""
 echo "✅ Font setup complete!"
+echo "Supported languages: Chinese (zh-CN/zh-TW), Japanese (ja), Korean (ko),"
+echo "                     Thai (th), Russian (ru), Vietnamese (vi),"
+echo "                     and all Latin-based languages (en, de, fr, es, etc.)"