# UI localisation and live language switching

PhotoCraft uses the same Rust catalog implementation in the native egui shell and the
WebAssembly build. English source strings are lookup keys. UTF-8 TSV catalogs supply plain,
contextual, command-ID and plural translations; missing entries fall back to English.

## Supported languages

| Language | Preference code | Integer plural forms |
|---|---|---|
| English | `en` | one / other |
| 简体中文 | `zh-hans` | one form |
| 日本語 | `ja` | one form |
| 한국어 | `ko` | one form |
| Русский | `ru` | one / few / many |
| Français | `fr` | singular for 0 and 1 / plural for 2+ |
| 繁體中文 | `zh-hant` | one form |
| Español | `es` | one / other |
| Čeština | `cs` | one / few / other |
| Deutsch | `de` | one / other |
| Bahasa Indonesia | `id` | one form |
| Português (Brasil) | `pt-br` | singular for 0 and 1 / plural for 2+ |

Every non-English catalog covers the current menu labels, `tl!` literals, blend mode names,
and generated preference labels. Tests enforce that coverage. This does not include every
engine error or status message: those remain English, as do automation command IDs and
user-provided document, layer and preset names.

## Switch without restarting

Open **Edit > Preferences > Interface > Language** and choose a language. The Preferences
dialog previews its title, controls and buttons in the chosen language. **Cancel** discards
the preview, **Apply** commits it without closing the dialog, and **OK** commits and closes.
The rest of the interface follows the committed preference on its next draw. Existing
documents, undo history and tools remain available throughout a language change.

The existing preference store saves the selection for future launches. `auto` follows the
native system locale; an unsupported code follows the same fallback. Regional tags such as
`fr-CA`, `ko-KR` and `zh-CN` resolve to the corresponding registered catalog. Traditional and
Simplified Chinese remain distinct. Every Portuguese locale (`pt`, `pt-BR`, `pt-PT`) uses the
Brazilian Portuguese catalog.

Agents use the existing `prefs.set` command through CLI, MCP or the desktop control channel:

```json
{"method":"engine.execute","params":{"command":"prefs.set","params":{"path":"interface.language","value":"ko"}}}
```

Control requests are processed before synchronising the drawing language, so a request
updates the labels in the same rendered frame. Drawing language is local to the UI thread;
a scoped preview restores the previous language even if drawing unwinds. Menu and dialog
identity continues to use command IDs, independent of translated labels.

This is hot switching among the bundled catalogs. Catalog files are embedded with
`include_str!`; editing a TSV file requires a rebuild. External catalog watching and
file-resource hot reload are not implemented.

## Fonts

The native lazy CJK font loader prioritises the selected Japanese, Korean or Chinese script.
Switching languages rebuilds the font definitions and resets the existing loader, so fonts
removed from the definitions can be registered again. Idle frames keep their font cache.
Other languages retain the system's CJK fallback preference for document names and text.

Font files are not added to this repository. Native rendering uses installed system fonts
and the optional craft-fonts build input; see [Fonts](development.md#fonts-craft-fonts).
The web build can select every catalog, but CJK glyph delivery and automatic browser-locale
detection remain separate outstanding work. Switching catalogs does not supply missing fonts.

## Add or maintain a catalog

1. Add `<code>.tsv` under `crates/ui-egui/src/i18n`, following the header in `ja.tsv`.
2. Register its code, native name and plural rule in `i18n::LANGUAGES`.
3. Translate the meaning of the English source keys. Preserve `{name}` placeholders, escaped
   characters and the trailing `…` on dialog commands. Use `@id` to disambiguate a command
   label and `@plural` for count-sensitive text. Keep product names such as PhotoCraft.
4. Set `complete_menus` when the catalog covers all enforced keys. Add source and license
   attribution for translation assets, then run the catalog and shell tests.

```sh
cargo test -p photocraft-ui-egui --locked
cargo clippy -p photocraft-ui-egui --all-targets --locked -- -D warnings
cargo xtask i18n-coverage
cargo xtask layers
cargo xtask wasm
```

Render representative dialogs with the offscreen `snapshot` example, including a sequence
of language changes in one process. Inspect the PNGs for missing glyphs and truncated labels.
Translations are original work or reuse the open PhotoCraft contributions credited in
[ATTRIBUTION.md](../ATTRIBUTION.md); no proprietary localisation resources are used.