diff --git a/api/core/dify_builder/models.py b/api/core/dify_builder/models.py
index 77a32493ac6..4106e786287 100644
--- a/api/core/dify_builder/models.py
+++ b/api/core/dify_builder/models.py
@@ -243,6 +243,7 @@ class DifyBuilderContext:
 
     # -- Build fields (Slice 2, additive) --
     goal_text: str = ""
+    reply_language: str = ""  # detected user-input language (BCP-47); drives reply/localization
     requirements: dict[str, Any] = field(default_factory=dict)
     plan_items: list[str] = field(default_factory=list)
     plan_version_tag: str = ""
diff --git a/api/core/dify_builder/runner.py b/api/core/dify_builder/runner.py
index ec81643f977..ce4b0626b1a 100644
--- a/api/core/dify_builder/runner.py
+++ b/api/core/dify_builder/runner.py
@@ -107,6 +107,10 @@ class Env:
     # the current operation and persisted with its assistant turn, independent
     # from observable execution progress.
     emit_reasoning: Callable[[ReasoningEventData], None] | None = None
+    # Detects the user's input language (BCP-47) for a turn's user text. None -> no detection.
+    detect_language: Callable[[str], str] | None = None
+    # Localizes an about-to-be-committed item list into the given language. None -> no-op.
+    localize_items: Callable[[list["ConversationItem"], str], list["ConversationItem"]] | None = None
     # Correlation metadata for the handler transition currently running.
     # The runner resets it before every independently committed step.
     session_id: str = ""
@@ -180,6 +184,8 @@ class Runner:
         settled: bool,
     ) -> None:
         at_version = session.version + 1
+        if self._env.localize_items is not None and context.reply_language:
+            items = self._env.localize_items(items, context.reply_language)
         for item in items:
             # Handler helpers own conversation sequence numbers; the runner is
             # the only layer that knows which CAS version will make them
@@ -251,6 +257,13 @@ class Runner:
         s, fc = self._env.repo.get_session(session_id)
         self._env.begin_operation(s)
 
+        if self._env.detect_language is not None:
+            user_text = turn.action.payload.get("text") if turn.action is not None else None
+            if isinstance(user_text, str) and user_text.strip():
+                fc.reply_language = self._env.detect_language(user_text)
+            elif not fc.reply_language and fc.goal_text:
+                fc.reply_language = self._env.detect_language(fc.goal_text)
+
         action_kind = turn.action.kind if turn.action is not None else ""
 
         # A retried message may arrive after its user bubble committed but the
diff --git a/api/tests/unit_tests/core/dify_builder/test_reply_language_engine.py b/api/tests/unit_tests/core/dify_builder/test_reply_language_engine.py
new file mode 100644
index 00000000000..ffead985e76
--- /dev/null
+++ b/api/tests/unit_tests/core/dify_builder/test_reply_language_engine.py
@@ -0,0 +1,65 @@
+"""Engine wiring: detect language from user input; localize items before commit."""
+
+from datetime import datetime
+
+from core.dify_builder.models import Action, Actor, DifyBuilderContext, EntryMode, Session, Turn
+from core.dify_builder.runner import Env, Runner
+from core.dify_builder.state import PcState
+from core.dify_builder.handlers_build import build_registry
+from core.dify_builder.handlers_edit import edit_registry
+from core.dify_builder.handlers_fix import fix_registry
+from tests.unit_tests.core.dify_builder.fakes import FakeDifyPort, InMemoryRepository
+from core.dify_builder.placeholder_agent import PlaceholderAgent
+
+
+def _env(**overrides):
+    repo = InMemoryRepository()
+    env = Env(dify=FakeDifyPort(), agent=PlaceholderAgent(), repo=repo, now=lambda: datetime.min, **overrides)
+    return env, repo
+
+
+def test_context_has_reply_language_default():
+    assert DifyBuilderContext().reply_language == ""
+
+
+def test_localize_items_called_on_commit_with_reply_language():
+    seen = {}
+
+    def localize(items, language):
+        seen["language"] = language
+        for it in items:
+            if it.payload.get("text") == "PING":
+                it.payload["text"] = "PONG"
+        return items
+
+    env, repo = _env(localize_items=localize)
+    runner = Runner(env, fix_registry() | build_registry() | edit_registry())
+    s = Session(app_id="app", tenant_id="t", owner_account_id="a",
+                entry_mode=EntryMode.FIX, current_state=PcState.FIX_AWAIT_DECISION)
+    fc = DifyBuilderContext(reply_language="zh-Hans")
+    repo.create_session(s, fc, [])
+    # a stop action produces a settled commit with no items -> localize still invoked
+    runner.advance(s.id, Turn(action=Action(kind="stop", base_version=1), actor=Actor(account_id="a", tenant_id="t")))
+    assert seen.get("language") == "zh-Hans"
+
+
+def test_detect_language_sets_reply_language_from_message_text():
+    detected = []
+
+    def detect(text):
+        detected.append(text)
+        return "ja"
+
+    env, repo = _env(detect_language=detect)
+    runner = Runner(env, fix_registry() | build_registry() | edit_registry())
+    s = Session(app_id="app", tenant_id="t", owner_account_id="a",
+                entry_mode=EntryMode.FIX, current_state=PcState.FIX_AWAIT_DECISION)
+    repo.create_session(s, DifyBuilderContext(), [])
+    runner.advance(
+        s.id,
+        Turn(action=Action(kind="message", payload={"text": "テスト", "client_turn_id": "t1"}, base_version=1),
+             actor=Actor(account_id="a", tenant_id="t")),
+    )
+    _, fc = repo.get_session(s.id)
+    assert fc.reply_language == "ja"
+    assert "テスト" in detected