Phase 5.9: Switch the interface between English and Spanish

[danielmrdev]

What to build

Make the complete Lettera interface available in English and Spanish from General Settings. A writer can follow the operating-system locale or choose a language, see the change immediately, and keep that choice across restarts.

Acceptance criteria

• Typed English and es-ES catalogs cover every user-visible interface message and tests reject missing or empty entries.
• System resolves Spanish operating-system locales to es-ES and all other locales to English.
• System, English, and Español choices apply live, persist through the versioned Preferences migration, and display language names in their own language.
• Untitled, errors, selector, conflict, Settings, Typeface, and titlebar text localize without changing Document content, File names, or Rust error codes.
• Locale changes do not alter the operating-system Spellcheck language.
• Development-gallery and component checks cover English and Spanish overflow in Paper and Ink.

Blocked by

• Phase 5.6: Browse, search, and open Files #42
• Phase 5.8: Deliver verified Typefaces through FontFace #44

Workspace

• Mode: integration
• Branch: feat/tauri-2-replatform
• Worktree: /home/daniel/Projects/lettera/.worktrees/feat-tauri-2-replatform
• Integrates into: feat/tauri-2-replatform
• Remote policy: local only — never push
• Completion: validate and commit directly in the integration worktree; no lane branch or worktree cleanup is required

[danielmrdev]

Frozen by the approved UI stack decision gate in #49. Do not implement this direct-migration ticket while the Floem/Tauri bake-off is open. Native dependency #54 records the live gate. After the blind decision, this ticket will be reviewed and respecified only if Tauri wins; if Floem wins, it will be closed as replaced.