[Feature Request] Add first-class Japanese language support (UI, docs, prompts)

What variant of Codex are you using?

CLI + IDE Extension + App

What feature would you like to see?

Japanese language support should be available as a first-class option across Codex surfaces.

Requested scope:

1. UI localization for key workflows (onboarding, settings, common actions, errors)
2. Official Japanese docs for core setup and troubleshooting
3. Prompt/template localization where applicable
4. Locale fallback strategy (ja-JP -> en)
5. Contribution guide for i18n updates

Why this matters:

• Lower onboarding friction for Japanese users
• Better support quality with fewer misunderstandings
• Stronger adoption in non-English-first teams

Additional information

I can help review terminology and prioritize high-impact strings first (MVP localization pass, then iterative expansion).