Browser language detection broken as soon as a request with no Accept-Language header happens

Browser language detection is skipped, if a browser/robot without Accept-Language header calls the page and the default language node is put into cache. Afterwards, the default language node is always delivered until cache is cleared/rebuilt, the browser language of subsequent requests is ignored.

My language detection order is:

• URL
• Browser
• Default / selected language

I built a test to illustrate this issue (drupal-browser-detection-test-2986325-1-D8.patch).

My solution for this issue is to trigger page_cache_kill_switch even if no Accept-Language header is set (but $this->languageManager and $request are not NULL). That is in the middle between the old code and the current.

LanguageNegotiationBrowser.php:

if ($this->languageManager && $request) {
      if ($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.
      \Drupal::service('page_cache_kill_switch')->trigger();
}