# Internationalization (i18n) — Goose Desktop UI

This document describes the i18n infrastructure for the Goose Desktop UI (`ui/desktop/`).

## Overview

The i18n system is built on [react-intl](https://formatjs.io/docs/react-intl/) (part of the FormatJS suite). It uses the **ICU MessageFormat** standard for translations, which provides full support for pluralization, gender/select, number/date formatting, and nested messages — all governed by CLDR rules.

**Key design decisions:**

- English strings live in source code as `defaultMessage` values — no duplication between code and catalog.
- The `@formatjs/cli` tool extracts messages automatically from source into translation catalogs.
- Date, time, and number formatting use the same locale as text translations (single source of truth via `IntlProvider`).
- No build pipeline changes required — react-intl is a pure runtime library.

## Marking strings for translation

### In React components

```tsx
import { defineMessages, useIntl } from 'react-intl';

const messages = defineMessages({
  greeting: {
    id: 'myComponent.greeting',
    defaultMessage: 'Hello, {name}!',
  },
  itemCount: {
    id: 'myComponent.itemCount',
    defaultMessage: '{count, plural, one {# item} other {# items}}',
  },
});

function MyComponent({ name, count }: { name: string; count: number }) {
  const intl = useIntl();
  return (
    <div>
      <h1>{intl.formatMessage(messages.greeting, { name })}</h1>
      <p>{intl.formatMessage(messages.itemCount, { count })}</p>
    </div>
  );
}
```

### Message ID conventions

Use dot-separated, hierarchical IDs that reflect the component location:

```
settings.appearance.title
sessions.delete.confirmMessage
launcher.placeholder
searchBar.caseSensitive
```

### ICU MessageFormat syntax

| Feature | Syntax | Example |
|---|---|---|
| Interpolation | `{variable}` | `Hello, {name}!` |
| Plural | `{var, plural, one {…} other {…}}` | `{count, plural, one {# file} other {# files}}` |
| Select | `{var, select, male {…} female {…} other {…}}` | `{gender, select, male {He} female {She} other {They}}` |
| Number | `{var, number}` | `{price, number, ::currency/USD}` |
| Date | `{var, date, medium}` | `{when, date, long}` |

The `#` symbol inside plural/selectordinal is replaced with the formatted number.

For full syntax details, see the [ICU MessageFormat specification](https://unicode-org.github.io/icu/userguide/format_parse/messages/).

## Extracting messages

After adding or modifying `defineMessages` calls, regenerate the English catalog:

```bash
cd ui/desktop
pnpm i18n:extract
```

This scans all `src/**/*.{ts,tsx}` files and writes the canonical English catalog to `src/i18n/messages/en.json`. Commit this file — it serves as the reference for translators.

### Keeping en.json in sync (automated check)

The `lint:check` script includes `i18n:check`, which re-runs extraction and verifies the output matches what's committed:

```bash
pnpm i18n:check
```

This runs as part of `pnpm lint:check` (and therefore CI). If a developer changes a `defaultMessage` in source but forgets to run `pnpm i18n:extract`, the check fails with a diff showing exactly what's out of date.

To compile messages into an optimized AST format (optional, for production performance):

```bash
pnpm i18n:compile
```

Compiled files go to `src/i18n/compiled/` (gitignored).

## Locale detection

The locale is resolved at startup in the following order:

1. **`GOOSE_LOCALE`** — explicit override (set on the `window` object or via env)
2. **`navigator.language`** — the browser/OS locale
3. **`"en"`** — fallback default

The resolved locale is used for both text translations and all Intl formatting (dates, numbers, relative times).

## Date and number formatting

### Inside React components

Use `intl.formatDate()`, `intl.formatNumber()`, `intl.formatRelativeTime()` from the `useIntl()` hook. These automatically use the same locale as text translations:

```tsx
const intl = useIntl();
intl.formatDate(new Date(), { month: 'long', day: 'numeric' });
intl.formatNumber(1234.5, { style: 'currency', currency: 'USD' });
```

### Outside React context

For utility functions that don't have access to the React tree (e.g., `timeUtils.ts`), import the resolved locale directly:

```ts
import { currentLocale } from '../i18n';
new Intl.DateTimeFormat(currentLocale, { ... }).format(date);
```

This ensures date/number formatting uses the same locale as the rest of the UI.

## Adding a new language

1. Copy `src/i18n/messages/en.json` to a new file, e.g., `src/i18n/messages/ja.json`.
2. Translate the `defaultMessage` values. Keep ICU syntax intact (e.g., `{count, plural, ...}`).
3. Add the locale code to `SUPPORTED_LOCALES` in `src/i18n/index.ts`.
4. Optionally run `pnpm i18n:compile` to pre-compile.

No other code changes are needed — `loadMessages()` dynamically imports the correct catalog at runtime.

## Testing

### Wrapping test renders with IntlProvider

Any component that uses `useIntl()` must be rendered inside an `IntlProvider`. Use the test helper:

```tsx
import { IntlTestWrapper } from '../i18n/test-utils';

render(<MyComponent />, { wrapper: IntlTestWrapper });
```

### i18n-specific tests

Unit tests for locale detection and message loading live in `src/i18n/i18n.test.ts`. Run them with:

```bash
cd ui/desktop
pnpm test:run -- src/i18n/i18n.test.ts
```

## Architecture summary

```
src/i18n/
├── index.ts          # Locale detection, loadMessages(), re-exports
├── messages/
│   └── en.json       # Extracted English catalog (committed)
├── compiled/         # Compiled catalogs (gitignored)
├── test-utils.tsx    # IntlTestWrapper for tests
└── i18n.test.ts      # Unit tests

src/renderer.tsx      # IntlProvider wraps the entire app tree
```