# Internationalization

Pi Web's interface uses a small internal i18n layer. It is intentionally kept
inside the application instead of introducing another runtime dependency, so
the UI can support more languages without coupling the server, API responses,
or Pi's own output to a particular language.

## Current Languages

The built-in language packages are:

- `en` - English
- `zh-CN` - Simplified Chinese

The initial locale is inferred from the browser. Users can change the locale
from the language control in the top bar. The selected locale is persisted in
`localStorage` under `pi-locale`.

## Using Translations In A Component

Client components should get the translation function from `useI18n`:

```tsx
import { useI18n } from "@/hooks/useI18n";

export function ExampleButton() {
  const { t } = useI18n();

  return <button aria-label={t("example.open")}>{t("example.open")}</button>;
}
```

The component must be rendered below `I18nProvider`. The application root
already provides it in `app/page.tsx`.

Use parameters for values that change at runtime instead of concatenating
translated fragments:

```tsx
<span>{t("session.messageCount", { count: messageCount })}</span>
```

The corresponding message can use `{count}`:

```ts
"session.messageCount": "{count} messages"
```

This keeps the sentence structure under the control of each language package.

## Adding A New Language

### 1. Create The Message File

Add a file under `lib/i18n/messages/`, using a BCP 47-style identifier. For
example, a Japanese package would be `lib/i18n/messages/ja.ts`:

```ts
import type { LocalePlugin } from "../types";

/** Pi Web Japanese language package. */
export const jaLocale: LocalePlugin = {
  id: "ja",
  label: "日本語",
  messages: {
    "common.language": "言語",
    // Add the rest of the shared keys here.
  },
};
```

The message keys must remain stable and language-neutral. Copy the complete
key set from `lib/i18n/messages/en.ts` when starting a new package. Missing
keys fall back to English, but a complete package is preferred for a user
facing language.

### 2. Register The Package

Import and register the package in `lib/i18n/registry.ts`:

```ts
import { jaLocale } from "./messages/ja";

registerLocale(jaLocale);
```

`LocalePlugin.id` must be unique and non-empty. The label is displayed in the
language selector. `getSupportedLocales()` automatically exposes registered
packages to the UI.

### 3. Update Locale-Specific Types If Needed

The built-in `Locale` type currently lists `en` and `zh-CN` for compile-time
safety. When adding a built-in language, update that union in
`lib/i18n/types.ts` and update any locale validation that intentionally lists
the built-in identifiers.

### 4. Add Tests

Extend `lib/i18n/registry.test.mjs` when browser-language detection or registry
behavior changes. Extend `lib/i18n/format.test.mjs` when interpolation or
locale-aware formatting changes. A new language should at minimum verify its
registry id and a representative translation.

## Adding Or Changing UI Text

1. Add a stable key to both `lib/i18n/messages/en.ts` and
   `lib/i18n/messages/zh-CN.ts`.
2. Put keys under a feature namespace such as `chat.*`, `files.*`,
   `settings.*`, or `common.*`.
3. Use `t("namespace.key")` in visible text, `title`, `aria-label`, and
   `placeholder` values.
4. Use interpolation for counts, names, paths, and other runtime values.
5. Keep product names, model names, provider names, commands, file paths,
   user content, tool output, and server-provided error details unchanged.
6. Do not translate an API error by matching its English text. Translate only
   the local UI fallback around it.

Keep established technical terms in English when translating them would make
the interface harder to map back to commands, configuration fields, or Pi
documentation. This includes terms such as `Agent`, `API Key`, `Provider`,
`Token`, `worktree`, `Diff`, `CWD`, `Shell`, `Git`, `HEAD`, `OAuth`, and
`Mermaid`; preserve their conventional casing. Translate the surrounding
actions and status text. Common concepts with clear localized forms, such as
model, plugin, skill, prompt, context, and cache, can remain localized.

For example:

```ts
// en.ts
"files.uploadedCount": "{count} uploaded"

// zh-CN.ts
"files.uploadedCount": "已上传 {count} 个文件"
```

```tsx
<span>{t("files.uploadedCount", { count: uploadedCount })}</span>
```

If a key is missing from the selected language, the formatter first tries the
English package and finally returns the key itself. Missing keys also produce
a development warning, which makes incomplete translations visible during
development.

## Verification

Run the following commands from the repository root:

```bash
node_modules/.bin/tsc --noEmit
npm run lint
node_modules/.bin/jiti lib/i18n/registry.test.mjs
node_modules/.bin/jiti lib/i18n/format.test.mjs
node --test lib/*.test.mjs
```

Do not run `next build` during normal development. See the repository
development notes for the reason.

## Design Principles

Pi Web is a user-facing web interface. Internationalization is therefore an
accessibility and usability feature: people should be able to understand and
operate the interface in a language they are comfortable with. Language
support should lower the barrier to using Pi Web, not be treated as a measure
of technical expertise or as a reason to exclude otherwise useful
contributions.

The i18n layer is deliberately incremental. Contributors can add a language
or improve a small group of UI messages without rewriting API contracts,
changing Pi's output, or introducing a large translation framework.