Add migration learning platform: Onboarding, Translation, Parent Portal

Phase 1.1 — user_language_api.py: Stores native language preference
per user (TR/AR/UK/RU/PL/DE/EN). Onboarding page with flag-based
language selection for students and parents.

Phase 1.2 — translation_service.py: Batch-translates vocabulary words
into target languages via Ollama LLM. Stores in translations JSONB.
New endpoint POST /vocabulary/translate triggers translation.

Phase 2.1 — Parent Portal (/parent): Simplified UI in parent's native
language showing child's learning progress. Daily tips translated.

Phase 2.2 — Parent Quiz (/parent/quiz/[unitId]): Parents can quiz
their child on vocabulary WITHOUT speaking DE or EN. Shows word in
child's learning language + parent's native language as hint.
Answer hidden by default, revealed on tap.

All UI text translated into 7 languages (DE/EN/TR/AR/UK/RU/PL).
Arabic gets RTL layout support.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend-lehrer/user_language_api.py

@@ -0,0 +1,86 @@
+"""
+User Language Preferences API — Stores native language + learning level.
+
+Each user (student, parent, teacher) can set their native language.
+This drives: UI language, third-language display in flashcards,
+parent portal language, and translation generation.
+
+Supported languages: de, en, tr, ar, uk, ru, pl
+"""
+
+import logging
+import os
+from typing import Any, Dict, Optional
+
+from fastapi import APIRouter, HTTPException, Query
+from pydantic import BaseModel
+
+logger = logging.getLogger(__name__)
+
+router = APIRouter(prefix="/user", tags=["user-language"])
+
+# Supported native languages with metadata
+SUPPORTED_LANGUAGES = {
+    "de": {"name": "Deutsch", "name_native": "Deutsch", "flag": "de", "rtl": False},
+    "en": {"name": "English", "name_native": "English", "flag": "gb", "rtl": False},
+    "tr": {"name": "Tuerkisch", "name_native": "Turkce", "flag": "tr", "rtl": False},
+    "ar": {"name": "Arabisch", "name_native": "\u0627\u0644\u0639\u0631\u0628\u064a\u0629", "flag": "sy", "rtl": True},
+    "uk": {"name": "Ukrainisch", "name_native": "\u0423\u043a\u0440\u0430\u0457\u043d\u0441\u044c\u043a\u0430", "flag": "ua", "rtl": False},
+    "ru": {"name": "Russisch", "name_native": "\u0420\u0443\u0441\u0441\u043a\u0438\u0439", "flag": "ru", "rtl": False},
+    "pl": {"name": "Polnisch", "name_native": "Polski", "flag": "pl", "rtl": False},
+}
+
+# In-memory store (will be replaced with DB later)
+_preferences: Dict[str, Dict[str, Any]] = {}
+
+
+class LanguagePreference(BaseModel):
+    native_language: str  # ISO 639-1 code
+    role: str = "student"  # student, parent, teacher
+    learning_level: str = "A1"  # A1, A2, B1, B2, C1
+
+
+@router.get("/languages")
+def get_supported_languages():
+    """List all supported native languages with metadata."""
+    return {
+        "languages": [
+            {"code": code, **meta}
+            for code, meta in SUPPORTED_LANGUAGES.items()
+        ]
+    }
+
+
+@router.get("/language-preference")
+def get_language_preference(user_id: str = Query("default")):
+    """Get user's language preference."""
+    pref = _preferences.get(user_id)
+    if not pref:
+        return {"user_id": user_id, "native_language": "de", "role": "student", "learning_level": "A1", "is_default": True}
+    return {**pref, "is_default": False}
+
+
+@router.put("/language-preference")
+def set_language_preference(
+    pref: LanguagePreference,
+    user_id: str = Query("default"),
+):
+    """Set user's native language and learning level."""
+    if pref.native_language not in SUPPORTED_LANGUAGES:
+        raise HTTPException(
+            status_code=400,
+            detail=f"Sprache '{pref.native_language}' nicht unterstuetzt. "
+                   f"Verfuegbar: {', '.join(SUPPORTED_LANGUAGES.keys())}",
+        )
+
+    _preferences[user_id] = {
+        "user_id": user_id,
+        "native_language": pref.native_language,
+        "role": pref.role,
+        "learning_level": pref.learning_level,
+    }
+
+    lang_meta = SUPPORTED_LANGUAGES[pref.native_language]
+    logger.info(f"Language preference set: user={user_id} lang={pref.native_language} ({lang_meta['name']})")
+
+    return {**_preferences[user_id], "language_meta": lang_meta}