OK, new results and now totally clueless!
I made a fresh clean emtpy 5.4.1 installation on my testing machine, added German as second language and made it to default language.
Everything is working fine, what does not really surprise
I compared the tables languages and portallanguages from this test-installation with my productive site and of course they are totally equal.
The only difference is that my prod enviroment was a 5.0 clean installation, filled with the content and then some updates instead of a 5.4.1 fresh installation.
But it is NOT working on my prod site. When clicking on the username the actual page will only be refreshed.
The URL behind the username is also the same in both worlds: javascrip:_doPostback('dnn$dnnUSER$cmdRegister','')
So, what the hell could this be???
Thanks in advance.
Cheers from the rainy Germany
Kai