lightcraft/docs/localization.md

# Localization

The interface ships in the language it is written in (English) plus every language in the table in
`crates/ui-egui/src/i18n.rs`. Today that is English, Simplified Chinese (`zh-hans`), Traditional
Chinese (`zh-hant`, Taiwan) and Japanese (`ja`). This file is the reference for **adding or maintaining a language**; the per-language notes
are in [`localization-zh-hans.md`](localization-zh-hans.md), [`localization-zh-hant.md`](localization-zh-hant.md) and
[`localization-ja.md`](localization-ja.md).

## Adding a language

1. **Translate the two catalogs.** Copy `locales/ja.json` and `locales/ja-formats.json` to
   `locales/<code>.json` and `locales/<code>-formats.json`, then translate the values. Keys are the
   English source strings and must stay byte-for-byte identical. The plain catalog may grow at its
   own pace: a message it lacks shows in English, and `cargo test -p lightcraft-ui-egui i18n::tests
   -- --nocapture` lists the gaps (it fails on an empty entry or mismatched placeholders). The
   formats catalog must carry every message (the build fails otherwise), including the date
   patterns (`{year}`, `{month} {year}`, `{weekday}, {day} {month} {year}`…) that date headings and
   capture times use in every language but English; weekday names go in the plain catalog.
2. **Add the table entry** in `crates/ui-egui/src/i18n.rs`:

   ```rust
   language_table! {
       En, "en";
       ZhHans, "zh-hans", "简体中文", "Hans", include_str!("../locales/zh-hans.json");
       ZhHant, "zh-hant", "繁體中文（台灣）", "Hant", include_str!("../locales/zh-hant.json");
       Ja, "ja", "日本語", "Jpan", include_str!("../locales/ja.json");
   }
   ```

   The fields are the BCP-47 code (also the settings-file value), the **endonym** shown in the
   Language menu, the ISO 15924 script (`Latn`, `Jpan`, `Hans`, `Hant`, `Kore`…), and the embedded
   catalog. `build.rs` picks up `<code>-formats.json` by name, so the macro and the format lookup
   follow automatically — nothing else in the UI needs to change. The menus, the settings row, the
   keyboard-shortcut sheet and the control channel all read the table.
3. **Make sure the glyphs exist.** The script drives which craft-fonts faces the UI installs: a
   language whose script has no face in the build shows boxes (the test says so instead of failing).
   Fonts live in [storytold/craft-fonts](https://github.com/storytold/craft-fonts), never here; a
   missing face is added there, not worked around in this repository.
4. Run `cargo xtask ci` (it includes `fmt`, `clippy`, the tests, `parity` and the wasm check).

## How a message is found

`tr("Exposure")` looks the English label up in the active language's catalog and returns the source
text unchanged when there is no entry; `tr_format!` handles text with runtime values. Both are
presentation-only: command ids, file names and user-entered metadata are never translated.

`tr_format!` is generated by `crates/ui-egui/build.rs`: one arm per English message, which formats
the active language's translation with `format!` (English is the fallback arm). Every catalog's
format string is therefore checked by Rust against the call site's own arguments:

```rust
tr_format!("Added {n} photo{} to “{}”", if n == 1 { "" } else { "s" }, album, n = n)
```

- A translation is an ordinary Rust format string over the same arguments. Refer to named values by
  name (`{n}`) and to positional ones by index when the word order changes (`{0}`, `{1}`):
  `"已将 {n} 张照片添加到“{1}”{0:.0}"`. Keep the source's spec (`{:.1}`, `{d:+}`, `{:.0}%`).
- `format!` rejects an unused argument, so a value a language does not need — the English plural
  suffix (`"s"` / `""`) — is consumed as `{:.0}` (a string at precision 0 prints nothing).
- Every formats catalog carries the same messages (`build.rs` fails the build otherwise), and a
  `tr_format!` call whose message is in no catalog does not compile: add it to every
  `*-formats.json` file.

## Testing and looking at it

```sh
cargo test -p lightcraft-ui-egui i18n::tests                       # catalogs, keys, placeholders, commands
CRAFT_FONTS_DIR=../craft-fonts cargo test -p lightcraft-ui-egui i18n::tests   # + glyph coverage
```

The glyph tests are skipped without `CRAFT_FONTS_DIR` (there are no CJK faces to check then).
To look at a language, render it headless:

```sh
LIGHTCRAFT_LANGUAGE=zh-hans lightcraft-cli snapshot --demo --script tour.jsonl -o out.png --size 1600x1000
```

or, with the app running (`--control 7980`), run the language command
(`engine.execute {"command": "app.language.simplifiedChinese"}`) and take a screenshot.

## Language codes and settings

The settings file stores the BCP-47 code, never the Rust variant name, so a language can be renamed
in code without invalidating anyone's saved preference. `LIGHTCRAFT_LANGUAGE` accepts what a system
locale looks like (`zh_Hans`, `zh-CN`, `en_US`, `ja_JP.UTF-8`) and falls back to the base language
when a region has no dedicated entry.