Possible bug in Lang::handleMissingKeysUsing()

[aarreedd]

I think I have stumbled across a small bug. Would like someone to confirm before I try creating a pull request to fix the problem.

Lang::handleMissingKeysUsing() is useful for logging missed translation keys. The third parameter of this callback is supposed to be the locale that is missing the translation. In some cases the $locale parameter is the config('app.fallback_locale') value instead of the current App::getLocale() value.

https://laravel.com/docs/10.x/localization#handling-missing-translation-strings

Steps to reproduce:

1. In your AppServiceProvider:

use Illuminate\Support\Facades\Lang;
 
/**
 * Bootstrap any application services.
 */
public function boot(): void
{
    Lang::handleMissingKeysUsing(function (string $key, array $replacements, string $locale) {
        dd($locale);
    });
}

2. In your config/app.php file, 'locale' => 'en' and 'fallback_locale' => 'en'.

3. In your routes/web.php:

Route::get('test', function() {
    app()->setLocale('fr');
    __('does not exist');
});

If the key does not exist in both fr and en then the $locale parameter of the Lang::handleMissingKeysUsing() function will be en. I would expect it to be fr.

This kind of makes sense if you are using "short keys". In that case, you should have a translation in your fallback_locale. But if you are using JSON file translation approach there is no need for a fallback locale.