Hi Stefano,
that's weird - saving the subscription settings page in every language (although there are no options at all) indeed fixed most of the problems. The message is now in the right language, the opt-in worked and the mail content also has the right language.
Only one thing is missing - the sender name. In whatever language I save it, it always overwrites all languages. I also looked in the database, it looks like this information is stored only once?
Thanks
Dominik