You'll need to set another one too. For example, to go to French from german
You can also set it on the command line
sqldeveloper.exe --AddVMOption=-Duser.language=en -Duser.country=us
Tools->Preferences->Database->NLS and change language and territory.
The problem was that I manipulated the wrong file .
The correct one is in C:\Users\<user>\AppData\Roaming\sqldeveloper\18.1.0.
<sqldeveloper> vs <SQL Developer> and <18.1.0.> vs <system18.1.0.095.1630>
Hard to confuse when i think about it, but I managed to confude them nevertheless.
Thanks for your quick help !