docs(i18n): locale fallback

Author: larbish

docs/content/en/2.concepts/6.internationalization.md

@@ -4,7 +4,8 @@ description: Create multi-language documentation with Docus v4 built-in i18n sup
 navigation:
   icon: i-lucide-globe
 seo:
-  description: Learn how to set up and manage multi-language documentation with Docus v4 built-in i18n support.
+  description: Learn how to set up and manage multi-language documentation with
+    Docus v4 built-in i18n support.
 ---
 
 Docus v4 introduces **native internationalization support** based on the `@nuxtjs/i18n` module, allowing you to create documentation in multiple languages with automatic routing and content management.
@@ -32,6 +33,7 @@ export default defineNuxtConfig({
   }
 })
 ```
+
 ::warning
 Docus overrides the `@nuxtjs/i18n` strategy to `prefix`.
 ::
@@ -48,7 +50,7 @@ npx create-docus my-docs -t i18n
 
 When i18n is enabled, organize your content by locale in the `content/` directory:
 
-```
+```text
 content/
 ├── en/                    # English content
 │   ├── index.md          # English homepage
@@ -66,7 +68,41 @@ content/
 │       └── advanced.md
 ```
 
-
 ::warning
 Each locale should mirror the same directory structure to maintain consistent navigation across languages.
 ::
+
+## Locale fallback
+
+Docus warns and skips any locale that does not exist in your `content/` directory. Missing locales are not registered.
+
+::prose-tip
+This is especially helpful when you extend Docus and use `@nuxtjs/i18n` for the rest of your site, but only want the docs in a subset of languages.
+::
+
+Docus detects locales from your nuxt config:
+
+```ts [nuxt.config.ts]
+export default defineNuxtConfig({
+  modules: ['@nuxtjs/i18n'],
+  i18n: {
+    locales: ['en', 'fr', 'ja'],
+    defaultLocale: 'en'
+  }
+})
+```
+
+But only register it based on your `content/` folder structure:
+
+```text
+content/
+├─ en/           # registered ✅
+├─ fr/           # registered ✅ (if present)
+└─ ja/           # skipped 🚫 (if missing)
+```
+
+If a user requests a missing docs locale, Docus redirects to the **default locale**.
+
+::prose-warning
+You must set a `defaultLocale` in your i18n config and ensure it exists under `content/<defaultLocale>`
+::