You just need a few lines of Javascript to automatically filter all your Views in Drupal.

Basically, we use the great Lazy Builder provided by Commerce Cart module. Easy peazy!

Menus are very specials in Drupal 8 - and sometimes very frustrated too!
Learn how to filter menu items by the current user's language in Drupal 8.

I a few word, it's been a mix of luck and sweat.