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。

未指定任何文件、测试或入口点。首先定位独立桌面应用 UI 和终端/CLI 集成区域，然后定义界面语言切换如何覆盖简体中文的菜单、设置、按钮和工具提示，同时将对话语言分开。