Hey Christian
Others have asked the same before, so there definitely seems to be a need for such a feature and personally also think it should be supported in the core. Can’t remember who did so in the past and how/if they solved it, hopefully they’ll chime in.
Can confirm it’s not possible out of the box, but can be achieved with some custom code.
The trick is to overwrite the FrontendNodeRoutePartHandler adding the logic you need.
This can be done since the routing is using an interface, see https://github.com/neos/neos/blob/master/Configuration/Routes.yaml#L63
To overwrite it create a custom class extending the default one and use Objects.yaml to overwrite the implementation class for the interface, example https://github.com/neos/neos/blob/master/Configuration/Objects.yaml#L45-L46
The context is build here https://github.com/neos/neos/blob/master/Classes/TYPO3/Neos/Routing/FrontendNodeRoutePartHandler.php#L240-L253
Everything related to the /de segment can be found by searching for uriSegment in the class.
The easiest solution is probably to remove the uriSegment setting from the dimension configuration completely and only rely on the domain for adding the language dimension.
Might be a bit tricky to achieve, but definitely doable.
For an estimate on how long it would take to get into the core, a rough estimate would be 2-3 days including the whole process of writing an RFC, getting feedback, implementing, documenting and the review process.
Also a related feature in development is RFC: Allow uriSegment to be empty for default presets