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Add a New Language

This guide describes how to add a new officially supported UI language.

Language support crosses the frontend and Rust app layer:

  • the frontend owns i18next resources and renders translated strings;
  • the Rust app layer owns first-run language fallback through SUPPORTED_LANGUAGES;
  • Settings persists the selected language through settings.json.

When adding a language, update both frontend and Rust support together.

Language Code

Use a stable short language code such as en, ja, or ru.

The current implementation stores this code directly in Settings and uses it as the i18next resource key. If a future language needs a regional code, check the Rust locale extraction logic in src-tauri/src/services/language_service.rs before choosing the code.

Steps

  1. Add src/lang/<code>.json.

    Use src/lang/en.json as the source structure. Keep the same key hierarchy and include lang.<code> so the Settings language selector can show the language name.

  2. Register the resource in src/lib/i18n.ts.

    Import the JSON file and add it to the resources object:

    import xx from "@/lang/xx.json";

const resources = {
  // existing languages...
  xx: {
    translation: xx,
  },
};

  3. Add the resource type in src/types/i18next.d.ts.

    Import the JSON type and add it to the Resources type.

  4. Add the language code to Rust SUPPORTED_LANGUAGES.

    Update src-tauri/src/services/language_service.rs so first-run OS language fallback recognizes the new language as officially supported.

  5. Update Rust language service tests.

    Add a test matching the existing supported_languages_contains_* tests in src-tauri/src/services/language_service.rs.

  6. Update frontend i18n tests.

    Add a resource-bundle assertion in src/lib/i18n.test.ts. If the new language intentionally relies on English fallback for some keys, add a focused fallback assertion.

  7. Check locale-sensitive formatting.

    Search for code that branches on settings.language. Some process and history table code currently chooses ja-JP or en-US explicitly. Update those call sites if the new language needs a locale-specific formatter.

  8. Check Settings UI.

    LanguageSelect derives available options from registered i18next resources. After step 2, the new language should appear automatically.

Validation

Run the relevant checks from the repository root:

npm test
cargo tauri-test

If the translation changes UI layout-sensitive text, manually inspect the Settings screen and any updated feature screens.