language per top-level domain not working

I tried to set up Polylang with two languages on two different top-level-domains. E.g. example.de and example.com
I have both domains on the same shared webspace in a multisite installation with sub-directories and Domain mapping.
Meaning: main-page.de/german (is mapped to example.de)
and main-page.de/english (is mapped to example.com)
Now I set up on both sites that “The language is set from different domains”
german: main-page.de/german
english: main-page.de/english
When I go into the german page, want to add an english translation to an existent post, the post get’s the URL slug: http://main-page.de/german/categoryname-en/post-title

What am I doing wrong here?