Flarum\Locale\LocaleManager::hasLocale PHP Method

    public function hasLocale($locale)
    {
        return isset($this->locales[$locale]);
    }

 /**
  * @param Request $request
  * @return Request
  */
 protected function logIn(Request $request)
 {
     $header = $request->getHeaderLine('authorization');
     $parts = explode(';', $header);
     $actor = new Guest();
     if (isset($parts[0]) && starts_with($parts[0], $this->prefix)) {
         $token = substr($parts[0], strlen($this->prefix));
         if (($accessToken = AccessToken::find($token)) && $accessToken->isValid()) {
             $actor = $accessToken->user;
             $actor->updateLastSeen()->save();
         } elseif (isset($parts[1]) && ($apiKey = ApiKey::valid($token))) {
             $userParts = explode('=', trim($parts[1]));
             if (isset($userParts[0]) && $userParts[0] === 'userId') {
                 $actor = User::find($userParts[1]);
             }
         }
     }
     if ($actor->exists) {
         $locale = $actor->getPreference('locale');
     } else {
         $locale = array_get($request->getCookieParams(), 'locale');
     }
     if ($locale && $this->locales->hasLocale($locale)) {
         $this->locales->setLocale($locale);
     }
     return $request->withAttribute('actor', $actor ?: new Guest());
 }