I heard a rumor Mode S transponders were compulsory.
AFAIK they aren't compulsory in Germany* if you are below 5000'. If you're routing through Holland, then you'll have to be below 1200' if you don't have Mode S. Also worth bearingh in mind that the Dutch now require you to have an ELT (not a PLB) if you are crossing their arispace.
Brooklands
*That was the situation when we went to Berlin last year.