(Late response as I was expecting to get an e-mail when someone replies)
Yes the language for the user is set to German (de-DE) in Joomla and Acymailing. I checked in the database and it's set correctly. When I send a test mail, it doesn't work correctly.
However, I just send the definite mail and then it DOES listen to the selected language. So the end result seems to work, it's just the test on the "Test" page (5th step, just before the summary page) that doesn't obey the language settings and thus makes you think it's not working/broken. So perhaps you guys can fix this specific page?
It would also be great when you do not have to change your admin account language for the Virtuemart product details to be send in the receivers language. The lack of this functionality basically kills the whole idea.