[Risolto] Changing languages does not work

Background of the issue:
I am trying to display a website in German & English, but only some of the pages in the menu are displayed in English, the others only in German. In the backend, the pages are translated correctly. Link to a page where the issue can be seen: link nascosto. I have recorded a video: link nascosto.

Symptoms:
When I click on the German page, only the German pages are displayed, although I am in English mode. I don't get any error messages.

Questions:
Why are only some pages displayed in English while others are only in German?
Why do German pages display even when I am in English mode?