In Laravel 13, where is the application's default locale configured, and how do you switch the locale for the current request?

## The configured default Every Laravel application has one **default locale**, the language used for translations when nothing else has chosen one. In the Laravel 13 skeleton it lives in `config/app.php`: ```php 'locale' => env('APP_LOCALE', 'en'), ``` So the usual way to change it is an entry in `.env`: ```ini APP_LOCALE=de ``` Because `env()` is called inside a config file, the value survives `php artisan config:cache`: the cached config already holds the resolved string. ## Switching at runtime `App::setLocale($locale)`, the facade call for the application's `setLocale()` method, changes the locale for the rest of the current request. In order, it: 1. writes the new value to the in-memory config key `app.locale`; 2. sets the translator's current locale, so later `__()` calls use it; the translator throws an `InvalidArgumentException` here for a value containing `/`, a backslash, `..` or a null byte; 3. dispatches `Illuminate\Foundation\Events\LocaleUpdated`, carrying the new `locale` and the `previousLocale`. Anything listening for that event can follow the change. Carbon's own Laravel service provider, which is package-discovered, listens for it and switches Carbon's locale to match. ## Reading the locale | Call | Returns | |---|---| | `App::currentLocale()` | the current value of `app.locale` | | `App::getLocale()` | the same value; `currentLocale()` is an alias | | `App::isLocale('de')` | `true` when the current locale equals the argument | | `App::getFallbackLocale()` | the value of `app.fallback_locale` | Use these rather than `config('app.locale')` so code reads as intent and survives a future change of storage. ## Why the choice does not persist Under PHP-FPM each request boots the application from scratch and reads `app.locale` from config again. `setLocale` changes memory only; it writes no cookie, session value or database row. A user's choice therefore has to be **stored somewhere** and **re-applied on every request**: typically a middleware reads a user column, a URL segment or the `Accept-Language` header and calls `App::setLocale()` before the controller runs. ## Validating input `setLocale` accepts any string without path characters, including codes you have no translations for, such as `xx`. The translator then misses every line and falls back, and pluralization rules for an unknown code choose the first form. Always check a user-supplied value against a list of supported locales before passing it on. ## Pitfalls - **`$request->setLocale('de')`** sets the locale property of the HTTP request object (inherited from Symfony's request class); it does not touch the translator, so `__()` output is unchanged. - **`config(['app.locale' => 'de'])`** writes config but, once the translator has been built, does not update it; only `App::setLocale()` does both and fires the event. - **Calling `env('APP_LOCALE')` in application code** returns `null` once config is cached, because `.env` is no longer loaded; read the config value or `App::currentLocale()` instead. - **Setting the locale in a controller** works for that action but is easy to forget elsewhere; a middleware keeps it in one place. ## Where it fits in a request A typical request on a multilingual site goes through these stages: 1. The application boots and the translator starts at `app.locale`. 2. A middleware decides the request's language from a stored preference, a URL segment or a header, and calls `App::setLocale()`. 3. Controllers, views, validation messages and anything else that translates now use that locale. 4. The response is sent and the in-memory state is discarded. Because stage 2 happens on every request, the decision must be cheap: read a column already loaded with the user, a route parameter or a header, not a remote service. ## Checking it on a running app - `php artisan about` lists the configured `Locale` among the environment details. - `php artisan config:show app` prints the whole `app` config, including `locale`, `fallback_locale` and `faker_locale`. - In a feature test, call `App::setLocale('de')` inside the test, request a page and assert the German text, so the expectation is explicit rather than dependent on `.env`.