feat(translation): add model, config, task, API, and UI for default document language translation

- Add detected_language, default_language_text, default_language_code columns to FileRecord
- Add default_document_language column to UserProfile
- Add DEFAULT_DOCUMENT_LANGUAGE config setting (defaults to "en")
- Create translate_to_default_language Celery task
- Integrate translation trigger into embed_metadata_into_pdf pipeline
- Add /api/files/{id}/translate and /api/files/{id}/translation/default API endpoints
- Add /files/{id}/text/default-language view endpoint
- Update file_view.html with translation sections (default language, on-the-fly)
- Add translation keys to en.json
- Create Alembic migration 036
- Update .env.demo

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

.env.demo

@@ -258,6 +258,13 @@ OPENAI_MODEL=gpt-4o-mini
 # AZURE_OPENAI_API_VERSION=2024-02-01
 # AI_MODEL=gpt-4o   # deployment name in Azure
 
+# **Document Translation**
+# After processing, documents whose detected language differs from the default
+# target language are automatically translated.  Only the original and this
+# default-language version are persisted; other translations are on-the-fly.
+# Users can override this in their profile settings.
+# DEFAULT_DOCUMENT_LANGUAGE=en
+
 # Azure Document Intelligence (OCR – separate from AI provider above)
 # **Email Settings (shared SMTP – password reset, verification, and system notifications)**
 EMAIL_HOST=smtp.example.com

app/api/__init__.py

@@ -43,6 +43,7 @@ from app.api.shared_links import public_router as shared_links_public_router
 from app.api.shared_links import router as shared_links_router
 from app.api.similarity import router as similarity_router
 from app.api.subscriptions import router as subscriptions_router
+from app.api.translation import router as translation_router
 from app.api.url_upload import router as url_upload_router
 
 # Import all the individual routers
@@ -96,3 +97,4 @@ router.include_router(audit_logs_router)
 router.include_router(i18n_router)
 router.include_router(mobile_router)
 router.include_router(compliance_router)
+router.include_router(translation_router)

app/api/translation.py

@@ -0,0 +1,158 @@
+"""
+API endpoints for document translation.
+
+Provides on-the-fly translation via the AI provider and access to the
+persisted default-language translation.
+"""
+
+import logging
+from typing import Annotated
+
+from fastapi import APIRouter, Depends, HTTPException, Query, Request, status
+from fastapi.responses import JSONResponse
+from sqlalchemy.orm import Session
+
+from app.auth import require_login
+from app.config import settings
+from app.database import get_db
+from app.models import FileRecord
+from app.utils.ai_provider import get_ai_provider
+from app.utils.user_scope import apply_owner_filter, get_current_owner_id
+
+logger = logging.getLogger(__name__)
+
+router = APIRouter()
+
+DbSession = Annotated[Session, Depends(get_db)]
+
+# Maximum characters sent to the AI provider for a single translation request.
+_MAX_TRANSLATION_INPUT = 50_000
+
+
+def _get_file_or_404(db: Session, file_id: int, request: Request) -> FileRecord:
+    """Fetch a FileRecord visible to the current user or raise 404."""
+    query = db.query(FileRecord).filter(FileRecord.id == file_id)
+    owner_id = get_current_owner_id(request, db)
+    if owner_id:
+        query = apply_owner_filter(query, owner_id, FileRecord)
+    record = query.first()
+    if not record:
+        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="File not found")
+    return record
+
+
+@router.get("/files/{file_id}/translation/default")
+@require_login
+def get_default_translation(
+    request: Request,
+    file_id: int,
+    db: DbSession,
+) -> JSONResponse:
+    """Return the persisted default-language translation for a document.
+
+    Returns 404 if no default-language translation has been generated yet
+    (e.g. because the document is already in the default language).
+    """
+    record = _get_file_or_404(db, file_id, request)
+
+    if not record.default_language_text:
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND,
+            detail="No default-language translation available for this file",
+        )
+
+    return JSONResponse(
+        content={
+            "file_id": record.id,
+            "detected_language": record.detected_language,
+            "default_language_code": record.default_language_code,
+            "text": record.default_language_text,
+        }
+    )
+
+
+@router.get("/files/{file_id}/translate")
+@require_login
+def translate_on_the_fly(
+    request: Request,
+    file_id: int,
+    db: DbSession,
+    lang: str = Query(..., min_length=2, max_length=10, description="Target language ISO 639-1 code"),
+) -> JSONResponse:
+    """Translate a document's extracted text into an arbitrary language on the fly.
+
+    The translation is generated via the configured AI provider and is **not**
+    persisted.  For the default-language translation, use the
+    ``/files/{file_id}/translation/default`` endpoint instead.
+    """
+    record = _get_file_or_404(db, file_id, request)
+
+    source_text = record.ocr_text
+    if not source_text:
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail="No extracted text available for this file — translation requires OCR text",
+        )
+
+    # If the requested language matches what is already stored, return it directly.
+    if record.default_language_code and lang == record.default_language_code and record.default_language_text:
+        return JSONResponse(
+            content={
+                "file_id": record.id,
+                "source_language": record.detected_language,
+                "target_language": lang,
+                "text": record.default_language_text,
+                "cached": True,
+            }
+        )
+
+    # If the detected language already matches, return the original text.
+    detected = record.detected_language
+    if detected and detected == lang:
+        return JSONResponse(
+            content={
+                "file_id": record.id,
+                "source_language": detected,
+                "target_language": lang,
+                "text": source_text,
+                "cached": True,
+            }
+        )
+
+    # Truncate to keep AI costs bounded.
+    text_to_translate = source_text[:_MAX_TRANSLATION_INPUT]
+
+    try:
+        provider = get_ai_provider()
+        model = settings.ai_model or settings.openai_model
+        translated = provider.chat_completion(
+            messages=[
+                {
+                    "role": "system",
+                    "content": (
+                        f"You are a professional translator. Translate the following text "
+                        f"into {lang}. Preserve the original formatting, paragraph structure, "
+                        f"and meaning. Do not add any commentary — output ONLY the translated text."
+                    ),
+                },
+                {"role": "user", "content": text_to_translate},
+            ],
+            model=model,
+            temperature=0.3,
+        )
+    except Exception as exc:
+        logger.exception(f"On-the-fly translation failed for file {file_id}: {exc}")
+        raise HTTPException(
+            status_code=status.HTTP_502_BAD_GATEWAY,
+            detail="Translation failed — the AI provider returned an error",
+        )
+
+    return JSONResponse(
+        content={
+            "file_id": record.id,
+            "source_language": detected or "unknown",
+            "target_language": lang,
+            "text": translated,
+            "cached": False,
+        }
+    )

app/config.py

@@ -166,6 +166,25 @@ class Settings(BaseSettings):
     google_docai_location: str = "us"  # Processor location, e.g. "us" or "eu"
     external_hostname: str = "localhost"  # Default to localhost
 
+    # ---------------------------------------------------------------------------
+    # Document Translation Settings
+    # ---------------------------------------------------------------------------
+    # Default target language for automatic document translation (ISO 639-1 code).
+    # After OCR / metadata extraction, if the detected document language differs
+    # from this value the system translates the extracted text into this language
+    # and stores it alongside the original.  Other language translations are
+    # generated on the fly via the AI provider and are NOT persisted.
+    # Per-user overrides are stored in UserProfile.default_document_language.
+    default_document_language: str = Field(
+        default="en",
+        description=(
+            "ISO 639-1 language code for the default translation target "
+            "(e.g. 'en', 'de', 'fr'). Documents whose detected language "
+            "differs are automatically translated into this language after "
+            "processing. Default: 'en' (English)."
+        ),
+    )
+
     # Authentication settings
     auth_enabled: bool = True  # Default to enabled
     admin_username: Optional[str] = None

app/models.py

@@ -84,6 +84,19 @@ class FileRecord(Base):
     # Processing pipeline assigned to this file (NULL = use system default)
     pipeline_id = Column(Integer, ForeignKey(_PIPELINES_ID_FK), nullable=True, index=True)
 
+    # Detected document language (ISO 639-1 code, e.g. "de", "en", "fr")
+    # Extracted from AI metadata during processing; cached here for fast access.
+    detected_language = Column(String(10), nullable=True)
+
+    # Default-language translation of the extracted text.
+    # Stored when the detected language differs from the user's/system default
+    # document language.  Only the original text and this translation are persisted;
+    # other languages are translated on the fly via the AI provider.
+    default_language_text = Column(Text, nullable=True)
+
+    # ISO 639-1 code of the default-language translation stored above (e.g. "en").
+    default_language_code = Column(String(10), nullable=True)
+
     # Timestamp when we inserted this record
     created_at = Column(DateTime(timezone=True), server_default=func.now(), index=True)
 
@@ -282,6 +295,12 @@ class UserProfile(Base):
     # NULL means "auto-detect from browser Accept-Language header"
     preferred_language = Column(String(10), nullable=True)
 
+    # Default document language for translated versions (ISO 639-1 code).
+    # When a document's detected language differs from this value, the system
+    # automatically generates and stores a translation into this language.
+    # NULL means "use the global DEFAULT_DOCUMENT_LANGUAGE setting".
+    default_document_language = Column(String(10), nullable=True)
+
     # UI colour scheme preference: "light" | "dark" | "system" (NULL = "system")
     preferred_theme = Column(String(10), nullable=True)
 

app/tasks/embed_metadata_into_pdf.py

@@ -216,6 +216,30 @@ def embed_metadata_into_pdf(self, local_file_path: str, extracted_text: str, met
                     except Exception as search_exc:
                         logger.warning(f"[{task_id}] Meilisearch indexing failed (non-fatal): {search_exc}")
 
+                    # Cache the detected language on the FileRecord and trigger
+                    # default-language translation when the document is in a
+                    # different language.
+                    detected_lang = metadata.get("language") if metadata else None
+                    if detected_lang and extracted_text:
+                        try:
+                            file_record.detected_language = detected_lang
+                            db.commit()
+
+                            from app.tasks.translate_to_default_language import translate_to_default_language
+
+                            translate_to_default_language.delay(
+                                file_id,
+                                extracted_text,
+                                detected_lang,
+                                owner_id=file_record.owner_id,
+                            )
+                            logger.info(
+                                f"[{task_id}] Queued default-language translation for file {file_id} "
+                                f"(detected: {detected_lang})"
+                            )
+                        except Exception as trans_exc:
+                            logger.warning(f"[{task_id}] Could not queue translation task (non-fatal): {trans_exc}")
+
         # Persist the metadata into a JSON file with the same base name.
         # Include file path references for traceability
         logger.info(f"[{task_id}] Persisting metadata to JSON")

app/tasks/translate_to_default_language.py

@@ -0,0 +1,141 @@
+#!/usr/bin/env python3
+"""Celery task to translate extracted document text into the default target language.
+
+This task is triggered after metadata extraction when the detected document
+language differs from the user's (or system) default document language.  The
+translated text is persisted in ``FileRecord.default_language_text`` so that
+users can always read a reference copy in their preferred language.
+
+Other ad-hoc translations are generated on the fly via the ``/api/files/{id}/translate``
+endpoint and are NOT persisted.
+"""
+
+import logging
+
+from app.celery_app import celery
+from app.config import settings
+from app.database import SessionLocal
+from app.models import FileRecord, UserProfile
+from app.tasks.retry_config import BaseTaskWithRetry
+from app.utils import log_task_progress
+from app.utils.ai_provider import get_ai_provider
+
+logger = logging.getLogger(__name__)
+
+
+def _resolve_default_language(owner_id: str | None) -> str:
+    """Return the default document language for the given owner.
+
+    Resolution order:
+    1. ``UserProfile.default_document_language`` (per-user override)
+    2. ``settings.default_document_language`` (global setting)
+    """
+    if owner_id:
+        with SessionLocal() as db:
+            profile = db.query(UserProfile).filter_by(user_id=owner_id).first()
+            if profile and profile.default_document_language:
+                return profile.default_document_language
+    return settings.default_document_language
+
+
+@celery.task(base=BaseTaskWithRetry, bind=True)
+def translate_to_default_language(
+    self,
+    file_id: int,
+    extracted_text: str,
+    detected_language: str,
+    owner_id: str | None = None,
+) -> dict:
+    """Translate *extracted_text* into the default document language and persist the result.
+
+    Args:
+        file_id: Primary key of the :class:`FileRecord`.
+        extracted_text: The OCR / refined text in the document's original language.
+        detected_language: ISO 639-1 code of the document's detected language.
+        owner_id: Owner identifier used to resolve per-user language preference.
+
+    Returns:
+        A dict with ``status``, ``target_language``, and the translated text length.
+    """
+    task_id = self.request.id
+    target_language = _resolve_default_language(owner_id)
+
+    # Nothing to do when the document is already in the target language.
+    if detected_language == target_language:
+        logger.info(
+            f"[{task_id}] Document {file_id} already in target language '{target_language}', skipping translation"
+        )
+        log_task_progress(
+            task_id,
+            "translate_to_default_language",
+            "skipped",
+            f"Document already in {target_language}",
+            file_id=file_id,
+        )
+        return {"status": "skipped", "reason": "already_in_target_language"}
+
+    logger.info(f"[{task_id}] Translating document {file_id} from '{detected_language}' to '{target_language}'")
+    log_task_progress(
+        task_id,
+        "translate_to_default_language",
+        "in_progress",
+        f"Translating from {detected_language} to {target_language}",
+        file_id=file_id,
+    )
+
+    try:
+        provider = get_ai_provider()
+        model = settings.ai_model or settings.openai_model
+        translated_text = provider.chat_completion(
+            messages=[
+                {
+                    "role": "system",
+                    "content": (
+                        f"You are a professional translator. Translate the following text "
+                        f"from {detected_language} to {target_language}. "
+                        f"Preserve the original formatting, paragraph structure, and meaning. "
+                        f"Do not add any commentary or explanation — output ONLY the translated text."
+                    ),
+                },
+                {"role": "user", "content": extracted_text},
+            ],
+            model=model,
+            temperature=0.3,
+        )
+
+        # Persist the translation.
+        with SessionLocal() as db:
+            record = db.query(FileRecord).filter_by(id=file_id).first()
+            if record:
+                record.default_language_text = translated_text
+                record.default_language_code = target_language
+                record.detected_language = detected_language
+                db.commit()
+                logger.info(
+                    f"[{task_id}] Stored default-language translation ({len(translated_text)} chars) for file {file_id}"
+                )
+
+        log_task_progress(
+            task_id,
+            "translate_to_default_language",
+            "success",
+            f"Translated {len(extracted_text)} → {len(translated_text)} chars ({detected_language} → {target_language})",
+            file_id=file_id,
+        )
+
+        return {
+            "status": "success",
+            "target_language": target_language,
+            "translated_length": len(translated_text),
+        }
+
+    except Exception as exc:
+        logger.exception(f"[{task_id}] Translation failed for file {file_id}: {exc}")
+        log_task_progress(
+            task_id,
+            "translate_to_default_language",
+            "failure",
+            f"Exception: {exc}",
+            file_id=file_id,
+        )
+        raise

app/views/files.py

@@ -832,6 +832,34 @@ def get_processed_text(request: Request, file_id: int, db: Session = Depends(get
         )
 
 
+@router.get("/files/{file_id}/text/default-language")
+@require_login
+def get_default_language_text(request: Request, file_id: int, db: Session = Depends(get_db)):
+    """Return the persisted default-language translation for the file view."""
+    from fastapi import status
+    from fastapi.responses import JSONResponse
+
+    from app.models import FileRecord
+
+    file_record = db.query(FileRecord).filter(FileRecord.id == file_id).first()
+    if not file_record:
+        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail=_FILE_NOT_FOUND)
+
+    if not file_record.default_language_text:
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND,
+            detail="No default-language translation available",
+        )
+
+    return JSONResponse(
+        content={
+            "text": file_record.default_language_text,
+            "language_code": file_record.default_language_code,
+            "detected_language": file_record.detected_language,
+        }
+    )
+
+
 @router.get("/duplicates")
 @require_login
 def duplicates_page(

frontend/templates/file_view.html

@@ -439,10 +439,105 @@
         </button>
       </div>
     </div>
+    {% if file.detected_language %}
+    <div style="font-size:0.75rem;color:#6b7280;margin-bottom:0.5rem;">
+      <i class="fas fa-globe" aria-hidden="true" style="margin-right:0.25rem;"></i>
+      Detected language: <strong>{{ file.detected_language }}</strong>
+    </div>
+    {% endif %}
     <div id="ocr-text-block" style="display:none;">
       <pre class="ocr-text" id="ocr-text-content">{{ file.ocr_text }}</pre>
     </div>
   </div>
+
+  <!-- ── Default-language translation ── -->
+  {% if file.default_language_text %}
+  <div class="doc-card">
+    <div style="display:flex;justify-content:space-between;align-items:center;margin-bottom:0.75rem;">
+      <div class="doc-card-title" style="margin:0;">
+        <i class="fas fa-language" aria-hidden="true" style="color:#3b82f6;margin-right:0.4rem;"></i>
+        Default Language Version
+        {% if file.default_language_code %}
+        <span style="font-size:0.75rem;color:#6b7280;font-weight:normal;margin-left:0.5rem;">({{ file.default_language_code }})</span>
+        {% endif %}
+      </div>
+      <div style="display:flex;gap:0.5rem;">
+        <button class="text-toggle" onclick="toggleDefaultLangText()" aria-expanded="false" aria-controls="default-lang-text-block">
+          <i id="default-lang-toggle-icon" class="fas fa-chevron-down" aria-hidden="true"></i>
+          <span id="default-lang-toggle-label">Show text</span>
+        </button>
+        <button class="text-toggle" onclick="copyDefaultLangText()" id="default-lang-copy-btn" style="color:#10b981;" aria-label="Copy default language text">
+          <i class="fas fa-copy" aria-hidden="true"></i> Copy
+        </button>
+      </div>
+    </div>
+    <div id="default-lang-text-block" style="display:none;">
+      <pre class="ocr-text" id="default-lang-text-content">{{ file.default_language_text }}</pre>
+    </div>
+  </div>
+  {% elif file.detected_language %}
+  <!-- Translation pending or document already in default language -->
+  <div class="doc-card">
+    <div style="display:flex;justify-content:space-between;align-items:center;">
+      <div class="doc-card-title" style="margin:0;">
+        <i class="fas fa-language" aria-hidden="true" style="color:#3b82f6;margin-right:0.4rem;"></i>
+        Default Language Version
+      </div>
+      <button class="action-btn btn-secondary" onclick="loadDefaultLangText({{ file.id }})">
+        <i class="fas fa-language" aria-hidden="true"></i> Load translation
+      </button>
+    </div>
+    <div id="default-lang-load-area" style="margin-top:0.75rem;"></div>
+  </div>
+  {% endif %}
+
+  <!-- ── On-the-fly translation ── -->
+  {% if file.ocr_text %}
+  <div class="doc-card">
+    <div style="display:flex;justify-content:space-between;align-items:center;margin-bottom:0.75rem;">
+      <div class="doc-card-title" style="margin:0;">
+        <i class="fas fa-exchange-alt" aria-hidden="true" style="color:#f59e0b;margin-right:0.4rem;"></i>
+        Translate to Another Language
+      </div>
+    </div>
+    <div style="display:flex;gap:0.5rem;align-items:center;flex-wrap:wrap;">
+      <label for="translate-lang-select" class="sr-only">Target language</label>
+      <select id="translate-lang-select" style="padding:0.4rem 0.6rem;border:1px solid #d1d5db;border-radius:0.375rem;font-size:0.85rem;min-width:160px;" aria-label="Select target language for translation">
+        <option value="">Select language…</option>
+        <option value="en">English</option>
+        <option value="de">Deutsch</option>
+        <option value="fr">Français</option>
+        <option value="es">Español</option>
+        <option value="it">Italiano</option>
+        <option value="pt">Português</option>
+        <option value="nl">Nederlands</option>
+        <option value="pl">Polski</option>
+        <option value="ru">Русский</option>
+        <option value="zh">中文</option>
+        <option value="ja">日本語</option>
+        <option value="ko">한국어</option>
+        <option value="ar">العربية</option>
+        <option value="hi">हिन्दी</option>
+        <option value="tr">Türkçe</option>
+        <option value="sv">Svenska</option>
+        <option value="da">Dansk</option>
+        <option value="no">Norsk</option>
+        <option value="fi">Suomi</option>
+        <option value="cs">Čeština</option>
+        <option value="ro">Română</option>
+        <option value="uk">Українська</option>
+      </select>
+      <button class="action-btn btn-secondary" onclick="translateOnTheFly({{ file.id }})" id="translate-btn" style="min-height:44px;min-width:44px;">
+        <i class="fas fa-language" aria-hidden="true"></i> Translate
+      </button>
+      <button class="text-toggle" onclick="copyTranslatedText()" id="translate-copy-btn" style="color:#10b981;display:none;" aria-label="Copy translated text">
+        <i class="fas fa-copy" aria-hidden="true"></i> Copy
+      </button>
+    </div>
+    <div id="translate-result-area" style="margin-top:0.75rem;"></div>
+  </div>
+  {% endif %}
+
   {% elif processed_file_exists or original_file_exists %}
   <div class="doc-card">
     <div style="display:flex;justify-content:space-between;align-items:center;">
@@ -688,6 +783,149 @@
     });
   }
 
+  // ── Default-language text toggle ──
+  function toggleDefaultLangText() {
+    var block = document.getElementById('default-lang-text-block');
+    var icon  = document.getElementById('default-lang-toggle-icon');
+    var label = document.getElementById('default-lang-toggle-label');
+    var btn   = icon ? icon.closest('button') : null;
+    if (!block) return;
+    if (block.style.display === 'none') {
+      block.style.display = 'block';
+      if (icon) icon.className = 'fas fa-chevron-up';
+      if (label) label.textContent = 'Hide text';
+      if (btn) btn.setAttribute('aria-expanded', 'true');
+    } else {
+      block.style.display = 'none';
+      if (icon) icon.className = 'fas fa-chevron-down';
+      if (label) label.textContent = 'Show text';
+      if (btn) btn.setAttribute('aria-expanded', 'false');
+    }
+  }
+
+  // ── Copy default-language text ──
+  function copyDefaultLangText() {
+    var content = document.getElementById('default-lang-text-content');
+    if (!content) return;
+    var text = content.textContent;
+    var btn = document.getElementById('default-lang-copy-btn');
+    if (!btn) return;
+    var orig = btn.innerHTML;
+    navigator.clipboard.writeText(text).then(function() {
+      btn.innerHTML = '<i class="fas fa-check"></i> Copied!';
+      setTimeout(function() { btn.innerHTML = orig; }, 2000);
+    }).catch(function() {
+      try {
+        var ta = document.createElement('textarea');
+        ta.value = text;
+        ta.style.cssText = 'position:fixed;opacity:0;';
+        document.body.appendChild(ta);
+        ta.select();
+        document.execCommand('copy');
+        document.body.removeChild(ta);
+        btn.innerHTML = '<i class="fas fa-check"></i> Copied!';
+        setTimeout(function() { btn.innerHTML = orig; }, 2000);
+      } catch(e) {}
+    });
+  }
+
+  // ── Load default-language text on demand ──
+  function loadDefaultLangText(fileId) {
+    var area = document.getElementById('default-lang-load-area');
+    if (!area) return;
+    area.innerHTML = '<div style="text-align:center;padding:1.5rem;color:#6b7280;"><i class="fas fa-spinner fa-spin fa-2x" aria-hidden="true"></i><p style="margin-top:0.5rem;">Loading translation…</p></div>';
+    fetch('/files/' + fileId + '/text/default-language')
+      .then(function(r) {
+        if (r.status === 404) throw new Error('No translation available yet — it may still be processing');
+        if (!r.ok) throw new Error('HTTP ' + r.status);
+        return r.json();
+      })
+      .then(function(data) {
+        area.innerHTML = '';
+        var info = document.createElement('div');
+        info.style.cssText = 'font-size:0.75rem;color:#6b7280;margin-bottom:0.5rem;';
+        info.textContent = 'Translated to: ' + data.language_code + (data.detected_language ? ' (from ' + data.detected_language + ')' : '');
+        area.appendChild(info);
+        var pre = document.createElement('pre');
+        pre.className = 'ocr-text';
+        pre.textContent = data.text;
+        area.appendChild(pre);
+      })
+      .catch(function(err) {
+        area.innerHTML = '<div style="color:#92400e;padding:0.75rem;background:#fef3c7;border-radius:0.375rem;font-size:0.875rem;"><i class="fas fa-info-circle" aria-hidden="true"></i> ' + err.message + '</div>';
+      });
+  }
+
+  // ── On-the-fly translation ──
+  var _translateCache = {};
+  function translateOnTheFly(fileId) {
+    var select = document.getElementById('translate-lang-select');
+    var area = document.getElementById('translate-result-area');
+    var copyBtn = document.getElementById('translate-copy-btn');
+    if (!select || !area) return;
+    var lang = select.value;
+    if (!lang) { area.innerHTML = '<div style="color:#92400e;padding:0.75rem;background:#fef3c7;border-radius:0.375rem;font-size:0.875rem;">Please select a target language.</div>'; return; }
+
+    // Check cache
+    if (_translateCache[lang]) {
+      renderTranslation(area, _translateCache[lang], lang);
+      if (copyBtn) copyBtn.style.display = '';
+      return;
+    }
+
+    area.innerHTML = '<div style="text-align:center;padding:1.5rem;color:#6b7280;" aria-live="polite"><i class="fas fa-spinner fa-spin fa-2x" aria-hidden="true"></i><p style="margin-top:0.5rem;">Translating…</p></div>';
+    if (copyBtn) copyBtn.style.display = 'none';
+
+    fetch('/api/files/' + fileId + '/translate?lang=' + encodeURIComponent(lang))
+      .then(function(r) { if (!r.ok) throw new Error('HTTP ' + r.status); return r.json(); })
+      .then(function(data) {
+        _translateCache[lang] = data.text;
+        renderTranslation(area, data.text, lang);
+        if (copyBtn) copyBtn.style.display = '';
+      })
+      .catch(function(err) {
+        area.innerHTML = '<div style="color:#dc2626;padding:0.75rem;background:#fee2e2;border-radius:0.375rem;font-size:0.875rem;"><i class="fas fa-exclamation-triangle" aria-hidden="true"></i> Translation failed: ' + err.message + '</div>';
+      });
+  }
+
+  function renderTranslation(container, text, lang) {
+    container.innerHTML = '';
+    var info = document.createElement('div');
+    info.style.cssText = 'font-size:0.75rem;color:#6b7280;margin-bottom:0.5rem;';
+    info.textContent = 'Translated to: ' + lang;
+    container.appendChild(info);
+    var pre = document.createElement('pre');
+    pre.className = 'ocr-text';
+    pre.id = 'translated-text-content';
+    pre.textContent = text;
+    container.appendChild(pre);
+  }
+
+  function copyTranslatedText() {
+    var content = document.getElementById('translated-text-content');
+    if (!content) return;
+    var text = content.textContent;
+    var btn = document.getElementById('translate-copy-btn');
+    if (!btn) return;
+    var orig = btn.innerHTML;
+    navigator.clipboard.writeText(text).then(function() {
+      btn.innerHTML = '<i class="fas fa-check"></i> Copied!';
+      setTimeout(function() { btn.innerHTML = orig; }, 2000);
+    }).catch(function() {
+      try {
+        var ta = document.createElement('textarea');
+        ta.value = text;
+        ta.style.cssText = 'position:fixed;opacity:0;';
+        document.body.appendChild(ta);
+        ta.select();
+        document.execCommand('copy');
+        document.body.removeChild(ta);
+        btn.innerHTML = '<i class="fas fa-check"></i> Copied!';
+        setTimeout(function() { btn.innerHTML = orig; }, 2000);
+      } catch(e) {}
+    });
+  }
+
   // ── Initialise on load ──
   document.addEventListener('DOMContentLoaded', function() {
     {% if (processed_file_exists or original_file_exists) and file %}

frontend/translations/en.json

@@ -1731,5 +1731,21 @@
   "upload.uploading": "Uploading...",
   "upload.url_description": "Enter a direct link to a file (PDF, Office documents, or images)",
   "upload.url_label": "File URL",
-  "upload.url_placeholder": "https://example.com/document.pdf"
+  "upload.url_placeholder": "https://example.com/document.pdf",
+
+  "translation.default_language_version": "Default Language Version",
+  "translation.detected_language": "Detected language",
+  "translation.show_text": "Show text",
+  "translation.hide_text": "Hide text",
+  "translation.copy": "Copy",
+  "translation.load_translation": "Load translation",
+  "translation.translate_to": "Translate to Another Language",
+  "translation.select_language": "Select language…",
+  "translation.translate_btn": "Translate",
+  "translation.translating": "Translating…",
+  "translation.no_translation": "No translation available yet — it may still be processing",
+  "translation.translated_to": "Translated to",
+  "translation.translation_failed": "Translation failed",
+  "translation.select_target": "Please select a target language.",
+  "translation.copied": "Copied!"
 }

migrations/versions/036_add_document_translation_fields.py

@@ -0,0 +1,44 @@
+"""Add document translation fields to files and user_profiles tables.
+
+Adds detected_language, default_language_text, and default_language_code to
+the files table so that a translated version of the document text can be
+stored alongside the original.
+
+Adds default_document_language to user_profiles so each user can override
+the system-wide default translation target language.
+
+Revision ID: 036_add_document_translation_fields
+Revises: 035_add_routing_rules
+Create Date: 2026-03-16
+"""
+
+from typing import Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "036_add_document_translation_fields"
+down_revision: Union[str, None] = "035_add_routing_rules"
+depends_on: Union[str, None] = None
+
+
+def upgrade() -> None:
+    """Add translation columns to files and user_profiles."""
+    with op.batch_alter_table("files") as batch_op:
+        batch_op.add_column(sa.Column("detected_language", sa.String(10), nullable=True))
+        batch_op.add_column(sa.Column("default_language_text", sa.Text(), nullable=True))
+        batch_op.add_column(sa.Column("default_language_code", sa.String(10), nullable=True))
+
+    with op.batch_alter_table("user_profiles") as batch_op:
+        batch_op.add_column(sa.Column("default_document_language", sa.String(10), nullable=True))
+
+
+def downgrade() -> None:
+    """Remove translation columns."""
+    with op.batch_alter_table("user_profiles") as batch_op:
+        batch_op.drop_column("default_document_language")
+
+    with op.batch_alter_table("files") as batch_op:
+        batch_op.drop_column("default_language_code")
+        batch_op.drop_column("default_language_text")
+        batch_op.drop_column("detected_language")