Fix: Topic word labels translate to selected language

Topics API now accepts lang= parameter. When lang=de, the word
labels are translated from English via Kaikki translations:
"eye, pupil, iris" → "Auge, Pupille, Iris"

Frontend sends searchLang to /topics endpoint and displays
display_words (translated) instead of words (English).

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

backend-lehrer/vocabulary/api.py

@@ -432,11 +432,15 @@ async def api_enrich_images(word_ids: List[str] = None):
 
 
 @router.get("/topics")
-async def api_get_topics(q: str = Query("", description="Search topic or word")):
+async def api_get_topics(
+    q: str = Query("", description="Search topic or word"),
+    lang: str = Query("en", description="Display language for word labels"),
+):
     """Find topics matching a search word. Returns related word lists.
 
     If q matches a topic name → returns that topic.
     If q matches a word in any topic → returns all topics containing that word.
+    Words are returned with translations if lang != en.
     """
     from vocabulary.db import get_pool
     pool = await get_pool()
@@ -451,10 +455,37 @@ async def api_get_topics(q: str = Query("", description="Search topic or word"))
                 ORDER BY word_count DESC
             """, f"%{q_lower}%", q_lower)
 
-    return {
-        "topics": [{"topic": r["topic"], "words": list(r["words"]), "word_count": r["word_count"]} for r in rows],
-        "query": q,
-    }
+        # Translate word labels if not English
+        topics = []
+        for r in rows:
+            en_words = list(r["words"])
+            display_words = en_words
+            if lang != "en":
+                # Batch-lookup translations from Kaikki
+                translated = []
+                for w in en_words[:20]:  # Limit to 20 for speed
+                    tr_row = await conn.fetchrow(
+                        "SELECT translations FROM vocabulary_kaikki WHERE lang = 'en' AND lower(word) = $1 LIMIT 1",
+                        w.lower(),
+                    )
+                    if tr_row and tr_row["translations"]:
+                        import json as _json
+                        tr = tr_row["translations"]
+                        if isinstance(tr, str):
+                            tr = _json.loads(tr)
+                        tr_text = tr.get(lang, {}).get("text", "")
+                        translated.append(tr_text if tr_text else w)
+                    else:
+                        translated.append(w)
+                display_words = translated + en_words[20:]
+            topics.append({
+                "topic": r["topic"],
+                "words": en_words,
+                "display_words": display_words,
+                "word_count": r["word_count"],
+            })
+
+    return {"topics": topics, "query": q, "lang": lang}
 
 
 class TranslateRequest(BaseModel):