Ok, another clean install with 2.3.1 on the live system. Dropped the db and removed all from the file system. A really clean install, only the i18n::set_locale('de_DE'); in the _config.php on mysite.
The result: interesting. With 2.3.1 all languages are functioning, only the german language do not ;-(. And with 2.3.1 german isn't functioning at all, no language mix, all is now in english, so the system don't find the german translations? I also removed the set_locale, but no chance, the german isn't functioning.
I also changed my installation: I uploaded this time the silverstripe tar to the server and extract it on the server. At the earlier installs, I extract the tar on my local machine and uploaded the complete directory tree with FTP. So there could be case problems with my earlier installs and its nothing to do with 2.3/2.3.1. But now the question is, why is german the only language that is not functioning ;-(
Another small observation: In the 2.3.1 installation under cms/lang there are strange files in the french translation fr_FR.php.orig,fr_FR.php.rej seems a merge problem.
So please one last idea, so I get the german language working ;-)? Is there a log file to see what is the problem with the german?