TypeScript#

The fallback locale is the canonical message schema:

const en = {
  common: { save: "Save" },
  users: { one: "{count} user", other: "{count} users" },
} as const;

const de = {
  common: { save: "Speichern" },
  users: { one: "{count} Benutzer", other: "{count} Benutzer" },
} satisfies Messages<typeof en>;

as const preserves nested keys and plural shape. satisfies Messages<typeof en> verifies another locale without requiring identical literal text.

createI18n() infers:

• valid locale names from messages
• valid translation keys from fallbackLocale
• eager initial/fallback locale requirements

const i18n = createI18n({ locale: "en", fallbackLocale: "en", messages: { en, de } });
i18n.t("common.save"); // valid
i18n.t("common.unknown"); // TypeScript error
i18n.setLocale("fr"); // TypeScript error

The Preact hooks cannot infer a provider’s generic type through context, so pass schema and locale types to useI18n() where strict key typing is required.