feat(brain): add OUTPUT_LANGUAGE reply-language lock

Add an optional OUTPUT_LANGUAGE env var that forces every reply into a
single language. When set, output_language_directive() injects a "respond
only in <language>" instruction (also forbidding other scripts) into the
chat loop's system prompt, next to the existing TTS English-only lock.
Empty (default) keeps the multilingual "reply in the user's language"
behaviour, so upstream is unaffected.

For the Korean-only deployment this also suppresses the occasional trailing
CJK/Hanja fragment qwen2.5:3b leaks on free-form chit-chat.

- system_prompt.py: language-agnostic output_language_directive() helper
- engine.py: read OUTPUT_LANGUAGE, append directive in _build_initial_system_message
- docker-compose.yml + .env.example: document/pass the new var
- docs/llm_contexts.md: note the new gating on the main reply context
- tests: cover unset/set/agnostic/whitespace cases

src/jarvis/reply/engine.py

@@ -5,10 +5,11 @@ Handles memory enrichment, tool planning and execution.
 """
 
 from __future__ import annotations
+import os
 from typing import Optional, TYPE_CHECKING
 
 from ..utils.redact import redact
-from ..system_prompt import build_system_prompt
+from ..system_prompt import build_system_prompt, output_language_directive
 from ..tools.registry import run_tool_with_retries, generate_tools_description, generate_tools_json_schema, BUILTIN_TOOLS
 from ..tools.builtin.stop import STOP_SIGNAL
 from ..debug import debug_log
@@ -1441,6 +1442,14 @@ def run_reply_engine(db: "Database", cfg, tts: Optional[Any],
                 "Always respond in English regardless of the language the user speaks in."
             )
 
+        # Deployment-level output-language lock (OUTPUT_LANGUAGE). When set,
+        # force every reply into that single language and forbid stray
+        # characters from other scripts. Empty (default) keeps the
+        # multilingual behaviour of replying in the user's own language.
+        _lang_directive = output_language_directive(os.environ.get("OUTPUT_LANGUAGE"))
+        if _lang_directive:
+            guidance.append(_lang_directive)
+
         if warm_profile_block:
             # Pre-query, query-agnostic user context. Lives OUTSIDE the
             # conversation-history section because it isn't a history

src/jarvis/system_prompt.py

@@ -6,6 +6,8 @@ who renames the wake word (e.g. "Friday") gets a butler with the matching
 name rather than a persona hardcoded to "Jarvis".
 """
 
+from typing import Optional
+
 _SYSTEM_PROMPT_TEMPLATE: str = (
     "Persona: you are a British butler named {name} — polite, composed, quietly amused, and "
     "quietly enjoying yourself. Default voice is dry, witty, and lightly sarcastic: you notice "
@@ -87,3 +89,26 @@ def build_system_prompt(assistant_name: str = "Jarvis") -> str:
     """
     name = (assistant_name or "Jarvis").strip() or "Jarvis"
     return _SYSTEM_PROMPT_TEMPLATE.format(name=name)
+
+
+def output_language_directive(language: Optional[str]) -> Optional[str]:
+    """Return a 'respond only in <language>' instruction, or None when unset.
+
+    Deployments that serve a single language set ``OUTPUT_LANGUAGE`` (read by
+    the reply engine). When it is empty/None the assistant keeps its default
+    multilingual behaviour of replying in whatever language the user wrote in,
+    so this returns ``None`` and no directive is injected.
+
+    The instruction is language-agnostic — it names whatever language string it
+    is given — and forbids mixing in other scripts. That exclusivity also
+    suppresses the occasional trailing CJK/Hanja fragment some small models
+    leak on free-form chit-chat.
+    """
+    lang = (language or "").strip()
+    if not lang:
+        return None
+    return (
+        f"Always respond only in {lang}, regardless of the language the user "
+        f"writes in. Do not mix in words, characters, or punctuation from any "
+        f"other language or script."
+    )