I didn't know that SMF2.1 still adheres to ANSI / ISO-8859-1.
Saving the files in utf8 format is not very helpful, because my text editor (Kate) then also changes the German special characters to rhombs with question marks. So the long way over search and replace! Ok ... just five minutes.
Was so free to give it an ID in the package-info. I think it's unique enough!
I replaced the attachment in the original post.
Now I also managed to upload the packages via manager. To do this, I have edited the language files again on a Windows system. For the utf8 encoded ones the line endings were set to Unix and not to Windows. I also packed them with WinRAR afterwards.
I'm just wondering if this is due to the package manager or the web server is a bit bitchy?