Wearing a helmet is mandatory for drivers of mopeds (construction speed 25 km/h and 45 km/h). So moped riders must also wear a helmet in Belgium and Germany. A bicycle helmet is not sufficient, it must be a moped helmet.
Felyx scooters in these cities therefore have a helmet in the top case or the buddy. So you don't need to bring your own helmet. The Felyx app also shows how many helmets there are when listing the scooter. In the other cities, wearing a helmet is not compulsory.