Make companion respond in the selected language (EN/FR)

The frontend now sends the current language to the backend.
The system prompt includes a language instruction so the LLM
responds in English or French matching the UI language.

https://claude.ai/code/session_015z3yZxNNfXF63JuQDuPbEG

app/llm.py

@@ -93,7 +93,13 @@ Produis un JSON strict avec cette structure :
 Réponds UNIQUEMENT avec le JSON, sans texte autour."""
 
 
-def build_system_prompt(mode: str, topic: str, phase: int, rag_chunks: list[str]) -> str:
+LANG_INSTRUCTION = {
+    "en": "\n\nIMPORTANT: You MUST respond entirely in English.",
+    "fr": "\n\nIMPORTANT: Tu DOIS répondre entièrement en français.",
+}
+
+
+def build_system_prompt(mode: str, topic: str, phase: int, rag_chunks: list[str], lang: str = "en") -> str:
     """Build the full system prompt with mode, phase guidance, and RAG context."""
     template = SYSTEM_TUTOR if mode == "TUTOR" else SYSTEM_CRITIC
 
@@ -104,6 +110,7 @@ def build_system_prompt(mode: str, topic: str, phase: int, rag_chunks: list[str]
               .replace("{phase}", str(phase)))
 
     prompt += f"\n\n{PHASE_GUIDANCE.get(phase, PHASE_GUIDANCE[0])}"
+    prompt += LANG_INSTRUCTION.get(lang, LANG_INSTRUCTION["en"])
 
     return prompt
 

app/main.py

@@ -42,6 +42,7 @@ class ChatRequest(BaseModel):
     topic: str = ""
     phase: int = 0
     phase_turns: int = 0
+    lang: str = "en"
     history: list[dict] = []
 
 
@@ -113,7 +114,7 @@ async def api_chat(req: ChatRequest):
         logger.info(f"Chat request: mode={req.mode}, topic={req.topic[:50]}, phase={req.phase}->{new_phase}, turns={req.phase_turns}->{new_phase_turns}, model={model}")
 
         rag_chunks = retrieve(req.message)
-        system_prompt = build_system_prompt(req.mode, req.topic, new_phase, rag_chunks)
+        system_prompt = build_system_prompt(req.mode, req.topic, new_phase, rag_chunks, req.lang)
 
         messages = [{"role": m["role"], "content": m["content"]} for m in req.history]
         messages.append({"role": "user", "content": req.message})

static/app.js

@@ -360,6 +360,7 @@
                 topic: state.topic,
                 phase: state.phase,
                 phase_turns: state.phaseTurns,
+                lang: currentLang,
                 history: state.history.slice(0, -1)
             })
         })
@@ -580,6 +581,7 @@
                 topic: state.topic,
                 phase: state.phase,
                 phase_turns: state.phaseTurns,
+                lang: currentLang,
                 history: []
             })
         })