Turn off Twig cache in general
In Drupal cache content is language specific. Default is a cache context for the language. Sometimes you need to clear the cache one more time, but then this should work.

You can check this in the database table cache_render. All cid's should contain [languages:language_interface]=langcode and if you visit the same page in different languages you should see multiple cache entries.

Disabling the cache with a null backend is only for development. For production you need to find a solution for the language specific caching issue.