[FEATURE]: Russian (ru) localization / i18n support for the TUI interface

[o9209082-star]

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

The opencode TUI interface is currently available only in English. I'd like to add internationalization (i18n) / localization support so the interface can be displayed fully in Russian (and potentially other languages).

What to change or add:

• Add an i18n framework / locale system to the TUI (button labels, menus, prompts, error messages).
• Add a language setting (e.g. in tui.json or via an environment variable) to select the interface language.
• Provide a Russian (ru) translation file.

Benefits:

• Makes opencode accessible to Russian-speaking users who are not comfortable with an English-only terminal interface.
• The documentation site (opencode.ai) already supports Русский, so extending the same language to the TUI would be consistent.

Note: the existing similar issue #27445 is closed, so this request is submitted as a fresh proposal. Thank you!

[github-actions]

Thanks for updating the issue! It now follows the contributing guidelines. We'll take a look.