Hello,
thank you for your offer. I found the problem in the meantime. The Language File was corrupt and I had to create a new one. now everything is working fine.
The problem with not been able to create a translation file was that the plugin “Disable WP Notification” blocked all the informations on the screen. After deactivating it, everything was working like it should be.
Thx again for your support.
Greetings
Matthias