UI localisation and live language switching

Picto 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

Languages registered with complete_menus cover the current menu labels, tl! literals, blend mode names, and generated preference labels. A string a catalog lacks shows in English: cargo xtask i18n-coverage lists the missing strings per language, and cargo xtask ci prints that list as its last step without failing. Vietnamese (vi) is selectable, resolves regional tags such as vi-VN, and covers all current stable localization keys. complete_menus remains false while generated/dynamic long-tail labels are still under review. 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, vi-VN 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. The Serbian catalog is written in Latin script (Ekavian); every Serbian locale (sr-RS, sr-Latn-RS, sr_RS@latin, and Cyrillic sr-Cyrl-RS) resolves to it.

First launch and system language

With no saved preference, interface.language defaults to auto, so the first rendered window uses the first supported system UI language. An older preferences file without this field also uses Auto. If detection fails or none of the preferred languages is registered, the UI uses English (en). A saved manual language choice takes precedence on later launches; startup never overwrites it or writes the detected language into the preference file.

The native-only, pinned sys-locale dependency provides safe Rust access to the platform APIs:

• Windows: GetUserPreferredUILanguages, in the user's preferred order. This uses UI languages rather than the Region setting for dates and numbers; they can differ.
• macOS: CFLocaleCopyPreferredLanguages, in preferred order, without starting defaults or depending on a particular .plist representation.
• Linux/BSD: the platform's standard locale environment, including LANGUAGE lists.

PICTO_LOCALE is an optional single-tag override for reproducible launches; an unsupported override falls back to English. Empty overrides use the system. Unix locale variables do not override Windows/macOS UI-language preferences. Tags are read once per process (at most 64, 128 bytes each) and matched against the supported language registry. Changes to the OS language list itself take effect at the next launch. The web build keeps its existing English Auto fallback until browser-locale detection is implemented.

API implementation and licensing: sys-locale.

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

{"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.

Right-to-left languages

Arabic, Hebrew, Persian, Urdu and the other right-to-left languages listed in i18n::dir_for_code mirror the window's chrome when chosen: the menu bar starts at the right with File, the toolbox docks on the right and the panel dock and its rail on the left, and the title, options and status bars read from the right (crates/ui-egui/src/direction.rs). The direction follows the language code, so a catalog needs no extra setting.

Not mirrored yet: panel interiors, dialogs, menu rows (shortcuts stay on the right), submenus (they open to the right), document tabs and the canvas. Before an Arabic catalog reads correctly, the UI also needs an Arabic fallback font (the bundled UI fonts have no Arabic glyphs) and bidirectional word order: egui 0.36 shapes each right-to-left word with HarfRust, so letters join, but lays a line's words out left to right, and its caret indexing is off in right-to-left text (emilk/egui#8576, #8577). Document text is laid out by the text engine.

No right-to-left catalog ships yet. To check the mirrored layout with any language, launch with PICTO_UI_DIRECTION=rtl (or ltr to force left-to-right); it is read once per process and has no effect on the web build:

PICTO_UI_DIRECTION=rtl cargo run -p picto-ui-egui --example snapshot -- --out rtl.png

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. 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 Picto.
4. Set complete_menus once cargo xtask i18n-coverage lists nothing missing for the language. Add source and license attribution for translation assets, then run the catalog and shell tests.

cargo test -p picto-ui-egui --locked
cargo clippy -p picto-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 Picto contributions credited in ATTRIBUTION.md; no proprietary localisation resources are used.