Turn off Twig cache in general Drupal does cache Twig templates. Normally, it is a good thing, as long as you don't have a multilingual site. 

In my case every t()is not working as the template is cached in the wrong in English, giving German no chance to ever be outputted.

Is this the proper way to deal with t() calls?

 $settings['cache']['bins']['render'] = 'cache.backend.null'; 
$settings['cache']['bins']['dynamic_page_cache'] = 'cache.backend.null';


It seems a bit wrong. As I understand these settings are more for the development process, not for production. Also there really should be a setting to use caching in a language specific way.
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.