code: public function filter(RouteCollection $collection, Request $request): RouteCollection
      {
          $languages = $request->getLanguages();
  
          foreach ($collection->all() as $name => $route) {
              if ('.fallback' !== substr($name, -9) && ($this->prependLocale || '.root' !== substr($name, -5))) {
                  continue;
              }
  
              /** @var PageModel $pageModel */
              $pageModel = $route->getDefault('pageModel');
  
              if (
                  !$pageModel instanceof PageModel
                  || $pageModel->rootIsFallback
                  || \in_array($pageModel->rootLanguage, $languages, true)
                  || preg_grep('/'.preg_quote($pageModel->rootLanguage, '/').'_[A-Z]{2}/', $languages)
              ) {
                  continue;
              }
  
              $collection->remove($name);
          }
  
          return $collection;
      }
docs: {@inheritdoc}