I circumvented the problem like this:
TYPO3CR:
contentDimensions:
'language':
default: 'en_US'
defaultPreset: 'en_US'
presets:
'all': ~
'en_US':
label: 'German' <--------
values: ['en_US']
uriSegment: 'de' <--------
Got fed up with that crap and this is better than wasting even more time ...