Internationalization — opt-in translations

English (en-US) is the default source locale. Do not create or update
additional locale catalogs unless the app's agent-native.config.ts lists them
under translations.locales.

Rule

Visible framework/template UI copy belongs in the app's i18n catalog, not
inline in components. When you add or edit UI text, update the English source
catalog first, then update only the locale catalogs listed in
translations.locales, and run the i18n guard.

Catalogs

Templates use app/i18n/:

• en-US.ts is the canonical source tree and fallback.
• Other locale files keep the same non-plural keys and the same placeholders.
• index.ts exports an AgentNativeI18nCatalog with English bundled and
  non-English catalogs loaded by dynamic import.

Use BCP-47 filenames from the supported set: en-US, zh-CN, es-ES,
fr-FR, de-DE, ja-JP, ko-KR, pt-BR, hi-IN, ar-SA.

If an app only ships translations for a subset of that set, set
supportedLocales on the AgentNativeI18nCatalog to that subset. Without it,
<LanguagePicker /> lists every framework-supported locale regardless of
whether loadMessages can actually resolve it.

UI Pattern

• Wrap apps with AppProviders i18n={{ catalog: i18nCatalog }}.
• Read strings with useT() and keep keys stable.
• Sidebar apps should expose a /settings route in the app sidebar. Put
  <LanguagePicker /> in that settings page, usually in a Language or General
  section, and keep the header language icon only as a quick-access shortcut.
• Settings pages should include an "Agent settings" row/card that calls
  openAgentSettings() from @agent-native/core/client to open the right
  agent sidebar's Settings tab. Localize the title, description, and button.
• Use useFormatters() for dates, numbers, relative time, and lists instead
  of embedding formatted values in translation strings.
• Do not translate stable identifiers: action names, route names, enum values,
  app-state keys, database values, protocol fields, env var names, or provider
  names.

Plurals And Placeholders

Plural strings use i18next/CLDR suffixes. Do not force every locale to copy
English plural categories:

• English uses _one and _other.
• Chinese and Japanese usually use _other.
• Arabic uses _zero, _one, _two, _few, _many, and _other.

Keep interpolation placeholders identical across locales, such as
{{count}}, {{name}}, or ICU-style {count} arguments.

RTL

New UI should be RTL-safe. Prefer logical CSS utilities/properties such as
ms-*, me-*, ps-*, pe-*, start-*, end-*, text-start, and
text-end when direction matters. Avoid hardcoded left/right positioning for
new localized UI unless it is genuinely physical.

Verification

Run:

pnpm guard:i18n-catalogs

For broader changes, also run the affected template tests and pnpm typecheck.
Machine translation is only a starting point; high-visibility strings need
human review.