Add Chinese (zh-CN) UI localization for the GitHub Copilot desktop app

**Is your feature request related to a problem? Please describe.**
The GitHub Copilot desktop app (standalone agent app, and the terminal/CLI integration) currently only supports an English UI — menus, settings, buttons, and other interface text. There is no option to switch the interface language, even though the underlying model can converse fluently in Chinese.

**Describe the solution you would like**
Please add an official interface localization option, including Simplified Chinese (zh-CN), for the desktop app UI (menus, settings panel, buttons, tooltips, etc.), separate from the conversational/chat language which already works well in Chinese.

**Additional context**
This would primarily benefit Chinese-speaking users who use the assistant in Chinese but currently have to navigate an English-only interface. A similar request for Spanish command descriptions was raised in #3681, so there appears to be existing appetite for i18n/localization work in this tool.

中文说明：目前 GitHub Copilot 桌面端的界面（菜单、设置选项、按钮等）仅支持英文，没有切换界面语言的选项。希望官方能够提供包括简体中文在内的界面本地化支持，与已经支持良好的中文对话功能区分开来。

No files, tests, or entry points are named. Start by locating the standalone desktop app UI and terminal/CLI integration areas, then define how an interface-language switch would cover menus, settings, buttons, and tooltips in Simplified Chinese while keeping conversational language separate.

Written by the indexing model from the issue text.