Decode language information embedded in the request path.
@todo Refactor this entire method to inline the relevant portions of drupal_language_initialize(). See the inline comment for more details.
Symfony\Component\HttpKernel\Event\GetResponseEvent $event: The Event to process.
public function onKernelRequestLanguageResolve(GetResponseEvent $event) {
// drupal_language_initialize() combines:
// - Determination of language from $request information (e.g., path).
// - Determination of language from other information (e.g., site default).
// - Population of determined language into drupal_container().
// - Removal of language code from _current_path().
// @todo Decouple the above, but for now, invoke it and update the path
// prior to front page and alias resolution. When above is decoupled, also
// add 'langcode' (determined from $request only) to $request->attributes.
drupal_language_initialize();
}