Add Japanese to localization settings

[neubig]

What problem or use case are you trying to solve?

A fair number of people in Japan are using OpenHands, but Japanese is not included in the localization settings. We should add Japanese as a localization option, and add Japanese translations to all localization configs.

[openhands-agent]

OpenHands started fixing the issue! You can monitor the progress here.

[openhands-agent]

An attempt was made to automatically fix this issue, but it was unsuccessful. A branch named 'openhands-fix-issue-6066' has been created with the attempted changes. You can view the branch here. Manual intervention may be required.

Additional details about the failure:
While the AI has made significant progress by adding Japanese localization support to the documentation system, the original issue requested Japanese translations for "all localization configs." Based on the AI's last message, only a subset of files have been translated:

• navbar.json
• intro.mdx
• code.json
• options.json
• current.json

This appears to be just the basic framework and some core navigation elements, but not a complete translation of all localization configurations as requested in the original issue. To fully resolve this issue, we would need:

1. A complete inventory of all localizable content in the system
2. Confirmation that ALL localizable content has been translated to Japanese
3. Verification that the translations are accurate and natural-sounding Japanese

The current implementation provides the infrastructure for Japanese localization but appears to be incomplete in terms of full content coverage. The PR should either:
a) Be expanded to include all remaining translations
b) Be clearly marked as a partial implementation with follow-up issues created to track the remaining translation work

[neubig]

Bad fix, I'll try over in the OpenHands app.