[Feature Request] Add i18n support / Simplified Chinese (zh-CN) localization for Codex UI

Summary

Codex (both CLI and the desktop app) currently ships with an English-only UI. Menu items, buttons, settings labels, status messages, onboarding text, and error strings are all hard-coded in English. There is no language switcher and no translation bundle.

For non-English speakers — especially the large Simplified Chinese developer community — this raises the barrier to adoption even though the underlying assistant happily responds in Chinese when instructed via AGENTS.md.

Problem

• Users can make the assistant reply in Chinese, but the surrounding chrome (sidebar, settings, command palette, toasts, dialogs, tray menu, etc.) stays in English.
• System-level browser/OS translation tools work poorly on Electron surfaces and break interactive elements.
• Patching app.asar to swap strings is fragile and gets wiped on every update.

Proposal

1. Introduce an i18n layer (e.g. i18next or equivalent) for all user-facing strings in the desktop app and CLI.
2. Externalize existing English copy into a default en-US locale file.
3. Ship Simplified Chinese (zh-CN) as the first additional locale, with a clear path for community-contributed locales.
4. Add a Language setting in Preferences and an equivalent --lang / config option for the CLI; fall back to the OS locale on first launch.

Why zh-CN first

• Large active developer base and high Codex interest in the region.
• Well-established translation conventions for IDE/editor terminology (VS Code, JetBrains, GitHub Desktop) to align with for consistency.

Willing to help

Happy to contribute the initial zh-CN translation file and review terminology once the i18n scaffolding lands.

Environment

• Codex Desktop (latest)
• OS: Windows 11 / macOS / Linux (issue is platform-agnostic)