Locale fallback

When a template is missing for the requested locale (undefined in the dictionary), the provider walks a configured fallback chain before throwing. An empty string "" is a valid template and does not trigger fallback.

"localeFallback": {
  "en": null,
  "de-DE": "en",
  "de-CH": "de-DE",
  "it": "en"
}

• null — terminal locale (stop walking).
• Any other value — next locale to try, recursively.
• Cycles are rejected at provider construction and during resolution.

Codegen emits LOCALE_FALLBACK and extends the locale union. The generated createI18n() factory wires the map into the engine automatically.

Partial regional locales (for example en-US overriding only a subset of keys, falling back to en) are a common use case — you do not need to duplicate the full dictionary for every variant.

With delivery: "split-by-locale" or custom, codegen applies the same fallback rules when writing delivery JSON, so each artifact already contains the strings the runtime would resolve for that locale or area.

Use xndrjs-i18n-audit to measure coverage: missingDirectByLocale lists keys without a template for that locale (translator backlog); missingEffectiveByLocale lists keys that would still fail at runtime after walking the fallback chain. See Overview — Audit script.

If the chain exhausts without a template, t() throws and includes the full chain:

[i18n] Missing key or locale: "welcome" [de-CH] (fallback chain: de-CH → de-DE → en)