feat(i18n): localize start/help/menu, practice, words, import, reminder, vocabulary, tasks/stats for RU/EN/JA; add JSON-based i18n helper\n\nfeat(lang): support learning/translation languages across AI flows; hide translations with buttons; store examples per lang\n\nfeat(vocab): add source_lang and translation_lang to Vocabulary, unique constraint (user_id, source_lang, word_original); filter /vocabulary by user.learning_language\n\nchore(migrations): add Alembic setup + migration to add vocab lang columns; env.py reads app settings and supports asyncpg URLs\n\nfix(words/import): pass learning_lang + translation_lang everywhere; fix menu themes generation\n\nfeat(settings): add learning language selector; update main menu on language change

utils/i18n.py

@@ -0,0 +1,51 @@
+import json
+from pathlib import Path
+from functools import lru_cache
+from typing import Any, Dict
+
+
+FALLBACK_LANG = "ru"
+
+
+@lru_cache(maxsize=16)
+def _load_lang(lang: str) -> Dict[str, Any]:
+    base_dir = Path(__file__).resolve().parents[1] / "locales"
+    file_path = base_dir / f"{lang}.json"
+    if not file_path.exists():
+        # fallback to default
+        if lang != FALLBACK_LANG:
+            return _load_lang(FALLBACK_LANG)
+        return {}
+    try:
+        return json.loads(file_path.read_text(encoding="utf-8"))
+    except Exception:
+        return {}
+
+
+def _resolve_key(data: Dict[str, Any], dotted_key: str) -> Any:
+    cur: Any = data
+    for part in dotted_key.split("."):
+        if not isinstance(cur, dict) or part not in cur:
+            return None
+        cur = cur[part]
+    return cur
+
+
+def t(lang: str, key: str, **kwargs) -> str:
+    """Translate key for given lang; fallback to ru and to key itself.
+
+    Supports dotted keys and str.format(**kwargs) placeholders.
+    """
+    data = _load_lang(lang or FALLBACK_LANG)
+    value = _resolve_key(data, key)
+    if value is None and lang != FALLBACK_LANG:
+        value = _resolve_key(_load_lang(FALLBACK_LANG), key)
+    if value is None:
+        value = key  # last resort: return the key
+    try:
+        if isinstance(value, str) and kwargs:
+            return value.format(**kwargs)
+    except Exception:
+        pass
+    return value if isinstance(value, str) else str(value)
+