docs: Localization / i18n contributor guide

[joelpeace48-cell]

Background & current state

i18n is tracked (#319), but there's no contributor guide for adding languages/translations, which will slow community localization.

Goal

Author docs/I18N_GUIDE.md: how to add a language, the translation workflow, and RTL considerations.

Outline

• Framework + file structure (aligned with Frontend: Add internationalization (i18n) framework and initial language support #319); adding a locale; translation keys conventions; pluralization; RTL; testing a locale; review process.

Task breakdown

• Write docs/I18N_GUIDE.md.
• Add a sample locale as a worked example.
• Cross-link from CONTRIBUTING.

Acceptance criteria

• A contributor can add a locale by following the doc (sample locale added).

Verification

• Add a sample locale; review.

Difficulty: easy · Effort: S · documentation