Hello Bernhard!
This is a known restriction in PPM language handling. The Item description respectively the project description is not language independent when logging in with EN or DE. So you have to maintain the description in both languages in EN and DE.
At the moment only notes can be customized as language-independet
=> area 0007 (General Default Values)
=> name 0017 (Notes Language Independent)
=> Settings value = 'X'
best regards,
Peter