Re-Routing the basic domain/ to /en/

I have written this topic also in the German forum, but it seems nobody has a tip. So I try it here:

We would like to launch a Shopware 6 store that runs in English and German.
English should be the main language.

Currently the domain domain/en/ is set up for English and domain/de/ for German.
Unfortunately we are stuck with redirecting domain/ to domain/en/. Since with the htaccess redirect we tried, everything is always redirected to /en/, thus creating an infinite loop.

Can the issue be solved from shopware? Or with the correct htaccess redirects?
Currently xxx.de/ " Oops! An Error Occurred ## The server returned a „500 Internal Server Error“."