The definitions for date and time in German lang file have double %% where only one % should be used:
$string['strftimedm'] = '%%d.%%m';
$string['strftimedmy'] = '%%d.%%m.%%Y';
$string['strftimedmyw'] = '%%d.%%m.%%y (%%a)';
$string['strftimeshortdate'] = '%%d.%%m.%%Y';
Correct
$string['strftimedm'] = '%d.%m';
$string['strftimedmy'] = '%d.%m.%Y';
$string['strftimedmyw'] = '%d.%m.%%y (%a)';
$string['strftimeshortdate'] = '%d.%m.%Y';