Thanks for your reply. I have seen this answer.
It points to a setting in the Page Names configuration of the Language Support module. I think this is were my problem comes in: In order for the multi-site module to work, the "name" of the page needs to be the actual domain that redirects to this part of the multi-site environment for both languages.
So "somedomain.com" as name for both "/en/" and "/de/".
But in order to have Processwire redirect a user to a specific language version of the homepage, the "name" of the homepage needs to be that of a language, so "/de/" and "/en/" in my case.