For newer Laravel versions (where the lang folder is not in the resources anymore but in the project's base folder) and with some more functionality:

/**
     * @param array array of required name replacements, eg. when you want to replace the 'en_BR' with just 'en', put ['en_BR' => 'en']
     * @return array return an array of available locales, eg. ['en', 'fr']
     */
    private function getAllLocales(array $replacements = []): array { 
             
        $locales = collect(array_diff(
            scandir(base_path('lang')), 
            ['..', '.'] // remove these strings from the result
        ))->filter(fn($item) => !str_contains($item, ".")) // removes all the files (these typically contain a dot in their name)
        ->map(fn($item) => $replacements[$item] ?? $item)->toArray(); // replaces all the given names (as key) in the $replacements argument with the specified new name (as value)

        return array_values($locales);
    }