Thanks for the reply.
I have a moodledata/lang/de/assign.php file which has this line 507 of 579 lines:
$string['submissionstatementdefault'] = 'Diese Arbeit ist meine persönliche Leistung. Sofern ich irgendwo fremde Quellen verwendet habe, sind diese Stellen entsprechend gekennzeichnet.';
The problem you describe has been reported as MDL-54731. It seems that all languages are affected, not just German.
A workaround is to go to Site administration / Plugins / Activity modules / Assignment / Assignment settings and enter the submission statement in the required language then save changes.
Thanks for pointing out to the bug. Did the workaround, making everybody reading the submission statement in German. Hope the En. and Fr. "minority courses" do not mind.
I won't have time to look deeper in the short term. But what I don't understand is that Moodle has solved the multilanguage problem brilliantly, right from the beginning. Why does this single string gives trouble? Was there a local planning problem? Its name 'submissionstatementdefault' is a bit suspicious to me. All these strings are default, as long as nobody changes them, aren't they?