For 99.x% of the data, the german data is identical to the original Catalyst version. But there are a handful of items, that are deviant on purpose. To enable players of all versions to get their version of the data, a language setting is needed.
Genesis will always check if there are objects matching the current language settings of the application, before returning the default object.
In my view it would be better if language and rule set could be selected independent of each other. Some people may want to play with US rules and German language or DE rules and English language.
See linked alterantive named ticket.