fix(avatar): follow language changes each turn

digital-avatar-app/backend/routers/chat.py

@@ -478,6 +478,24 @@ def _qa_requires_language_adaptation(question: str, answer: str) -> bool:
     )
 
 
+def _qa_requires_per_turn_rendering(
+    question: str,
+    answer: str,
+    history: list[Any],
+) -> bool:
+    """Keep the direct QA fast path only when no conversation can bias language."""
+    return bool(history) or _qa_requires_language_adaptation(question, answer)
+
+
+def _per_turn_language_instruction() -> str:
+    return (
+        "本轮语言覆盖指令：只根据紧随其后的最新用户消息判断本轮回答语言。"
+        "即使此前整段对话一直使用另一种语言，只要最新消息切换了语言，本轮就必须立即切换到相同语言；"
+        "不要沿用上一轮语言。若最新消息明确指定回答语言，以该指定为准；若混用多种语言，使用其中占主导的"
+        "自然语言。不要说明你检测、切换或翻译了语言。"
+    )
+
+
 def _canonicalize_question(value: str) -> str:
     value = _normalize_question(value)
     replacements = (
@@ -709,6 +727,8 @@ def _build_prompt(
     messages = [{"role": "system", "content": system}]
     for item in history[-MAX_HISTORY_MESSAGES:]:
         messages.append({"role": item.role, "content": item.content} if hasattr(item, "role") else item)
+    # Keep the language instruction adjacent to the current turn so long histories cannot override it.
+    messages.append({"role": "system", "content": _per_turn_language_instruction()})
     messages.append({"role": "user", "content": question.strip()})
     return messages
 
@@ -845,7 +865,7 @@ def _resolve_reply(
         qa_pairs = db.query(QAPair).filter(QAPair.avatar_id == avatar.id).all()
     matched = _match_standard_qa(question, qa_pairs)
     adapt_qa_language = bool(
-        matched and _qa_requires_language_adaptation(question, matched.answer)
+        matched and _qa_requires_per_turn_rendering(question, matched.answer, history)
     )
     if matched and not adapt_qa_language and not image_contexts:
         return {"answer": matched.answer, "source": "qa", "references": []}
@@ -940,7 +960,7 @@ def _stream_reply(
     qa_pairs = db.query(QAPair).filter(QAPair.avatar_id == avatar.id).all()
     matched = _match_standard_qa(question, qa_pairs)
     adapt_qa_language = bool(
-        matched and _qa_requires_language_adaptation(question, matched.answer)
+        matched and _qa_requires_per_turn_rendering(question, matched.answer, history)
     )
     messages, reservation = [], None
     if matched and not adapt_qa_language and not image_contexts: