Feature/delta team/epic r04 (#7)

## Summary

epic r04 - begin refactor

## Change Type

- [x] Cowork feature
- [ ] Bug fix
- [ ] Core AI contribution
- [ ] Test / hardening
- [ ] Performance
- [ ] Documentation

## Related Work

Cowork Task:

Core Repo: http://34.143.229.138/gitea-admin/fsg-ai-core-assets

Core AI Issue:

Core Task:

Related PR:

## Scope

What is intentionally included?

What is intentionally NOT included?

## Validation

- [ ] Unit tests
- [ ] Integration tests
- [ ] Manual verification
- [ ] Regression check

Commands / evidence:

## Security Impact

Permission / credential / network / customer data impact:

## Compatibility

- [ ] No breaking change
- [ ] Breaking change documented

## Reviewer Notes

Anything Cowork reviewers should pay attention to.

---------

Co-authored-by: Anh Tran Nguyen Minh <anhtnm1@fpt.com>
Co-authored-by: Huong Le Thi Thien <huongltt35@fpt.com>
Co-authored-by: Nam Pham Dinh Thanh <nampdt@fpt.com>
Co-authored-by: Vu Dam Tuan <vudt15@fpt.com>
Co-authored-by: Hiep Ha Van <hiephv3@fpt.com>
Co-authored-by: Lam Hoang Van <lamhv7@fpt.com>
Reviewed-on: #7
Co-authored-by: Duy Le Huu <duylh19@fpt.com>

i18n/__init__.py

@@ -0,0 +1,101 @@
+"""Runtime UI translation: English / Japanese / Vietnamese.
+
+``tr(key, **kwargs)`` returns the string for the current language (falling
+back to English, then the key itself so a missing entry is still visible
+instead of crashing). ``.format(**kwargs)`` is applied when placeholders are
+passed, so callers can do e.g. ``tr("composer.attachments", n=3)``.
+
+Persistent, long-lived widgets (the main window chrome, the tabs, the
+sidebar, the composer, ...) must reflect a language change immediately, so
+they register a zero-arg callback via :func:`on_language_changed` that
+re-applies ``tr()`` to their own text; the callback runs once right away and
+again every time the language changes. Transient dialogs (Settings, Skills,
+Flow, Permission...) are rebuilt from scratch each time they are opened, so
+they simply call ``tr()`` while constructing their widgets and need no
+registration.
+"""
+from __future__ import annotations
+
+from typing import Callable, Dict, List
+
+LANGUAGES: Dict[str, str] = {"en": "English", "ja": "日本語", "vi": "Tiếng Việt"}
+# Short codes shown in the compact top-bar switcher (Settings keeps the full names above).
+LANGUAGE_SHORT: Dict[str, str] = {"en": "EN", "ja": "JP", "vi": "VN"}
+DEFAULT_LANGUAGE = "vi"
+
+_current = DEFAULT_LANGUAGE
+_listeners: List[Callable[[], None]] = []
+
+# key -> {"en": ..., "ja": ..., "vi": ...}
+from . import login_dialog as _login_dialog
+from . import sidebar as _sidebar
+from . import composer as _composer
+from . import hint as _hint
+from . import cowork_tab as _cowork_tab
+from . import settings_dialog as _settings_dialog
+from . import skills_dialog as _skills_dialog
+from . import libreoffice_view as _libreoffice_view
+from . import agents_admin_tab as _agents_admin_tab
+from . import monitoring_overview as _monitoring_overview
+
+# Gộp theo đúng thứ tự cũ: khoá trùng thì cụm sau thắng, y như khi tất cả
+# còn nằm chung một dict literal.
+STRINGS: Dict[str, Dict[str, str]] = {
+    **_login_dialog.STRINGS,
+    **_sidebar.STRINGS,
+    **_composer.STRINGS,
+    **_hint.STRINGS,
+    **_cowork_tab.STRINGS,
+    **_settings_dialog.STRINGS,
+    **_skills_dialog.STRINGS,
+    **_libreoffice_view.STRINGS,
+    **_agents_admin_tab.STRINGS,
+    **_monitoring_overview.STRINGS,
+}
+
+
+def set_language(lang: str) -> None:
+    """Switch the active language and notify every registered persistent widget."""
+    global _current
+    if lang not in LANGUAGES:
+        lang = DEFAULT_LANGUAGE
+    if lang == _current:
+        return
+    _current = lang
+    for fn in list(_listeners):
+        try:
+            fn()
+        except RuntimeError:
+            # The widget behind this callback was already destroyed — drop it.
+            try:
+                _listeners.remove(fn)
+            except ValueError:
+                pass
+
+
+def get_language() -> str:
+    """Mã ngôn ngữ đang dùng."""
+    return _current
+
+
+def tr(key: str, **kwargs) -> str:
+    """Chuỗi đã dịch cho một khoá.
+
+    Thiếu khoá thì trả về CHÍNH khoá đó — hiện ra một chuỗi lạ trên giao diện
+    vẫn tốt hơn là làm vỡ màn hình. Thiếu bản dịch của ngôn ngữ hiện tại thì rơi
+    về tiếng Anh.
+    """
+    entry = STRINGS.get(key)
+    if not entry:
+        return key
+    text = entry.get(_current) or entry.get("en") or next(iter(entry.values()), key)
+    return text.format(**kwargs) if kwargs else text
+
+
+def on_language_changed(fn: Callable[[], None]) -> None:
+    """Register a callback that re-applies translations to a persistent widget.
+
+    Called once immediately (to apply the current language) and again on every
+    future call to :func:`set_language`."""
+    _listeners.append(fn)
+    fn()