[ils15]

Closes #441

Summary

• Add shared English and Simplified Chinese locale catalogs with English fallback, and localize the Desktop GUI, TUI, shared formatters, and plugin-owned error text.
• Add automatic/explicit language selection and document how to contribute translations.
• Preserve the before/after screenshots in .github/pr-evidence/issue-441/ and embed direct links below.

Before / after captures

Desktop GUI	TUI
English Simplified Chinese	English Simplified Chinese

Verification

• PATH=/tmp/opencode/pnpm-gate-bin:$PATH pnpm run check — formatting and lint passed.
• PATH=/tmp/opencode/pnpm-gate-bin:$PATH pnpm test — 151 test files passed, 1 skipped; 3,974 tests passed, 2 skipped.
• PATH=/tmp/opencode/pnpm-gate-bin:$PATH pnpm run build — passed (Vite+ emitted an inlineDynamicImports deprecation warning).

Automated surfaces covered: locale/fallback and shared formatting; Desktop GUI usage/surface behavior; TUI settings, plugin tab, windows, status line, and toasts; core usage ranges and error wrappers.

Not manually verified: interactive OpenCode Desktop/TUI behavior on Windows, and locale selection through a live host-provided language setting. The captures are checked-in review evidence, not a substitute for that manual verification.

Integration note

#441 and #442 touch overlapping paths. #442's PR #444 is open; consider landing #442 first, then rebase/resolve #441 against the updated main if it advances. This branch is based on an earlier main commit.