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>

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