class LanguageNegotiationBrowser

class LanguageNegotiationBrowser extends LanguageNegotiationMethodBase implements ContainerFactoryPluginInterface {
    
    /**
     * The language negotiation method id.
     */
    const METHOD_ID = 'language-browser';
    
    /**
     * The page cache disabling policy.
     *
     * @var \Drupal\Core\PageCache\ResponsePolicy\KillSwitch
     */
    protected $pageCacheKillSwitch;
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
        $instance = new static();
        $instance->pageCacheKillSwitch = $container->get('page_cache_kill_switch');
        return $instance;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getLangcode(?Request $request = NULL) {
        $langcode = NULL;
        if ($this->languageManager && $request && $request->server
            ->get('HTTP_ACCEPT_LANGUAGE')) {
            $http_accept_language = $request->server
                ->get('HTTP_ACCEPT_LANGUAGE');
            $langcodes = array_keys($this->languageManager
                ->getLanguages());
            $mappings = $this->config
                ->get('language.mappings')
                ->get('map');
            $langcode = UserAgent::getBestMatchingLangcode($http_accept_language, $langcodes, $mappings);
        }
        // Internal page cache with multiple languages and browser negotiation
        // could lead to wrong cached sites. Therefore disabling the internal page
        // cache.
        // @todo Solve more elegantly in https://www.drupal.org/node/2430335.
        $this->pageCacheKillSwitch
            ->trigger();
        return $langcode;
    }

}