European Community regulation EU 2018/1139 set the rules to operate any aircraft in Europe.
The Annex number I of this regulation (page L212/85) set the aircraft categories which don’t have to comply with those rules (airworthiness, pilot license, maintenance approval, etc.).
Among other categories, like warbirds, vintage airplanes, kit planes, ultralights are mentioned at § e):
aircrafts having measurable stall speed or the minimum steady flight speed in landing configuration not exceeding 35 knots calibrated air speed (CAS), and a maximum take-off mass (MTOM) of no more than:
As a result of those exemptions, each european state is free to set it’s own regulations for ultralights within this mass limitations. They almost did it all for the motrized ultralights, but for the gliders its more complicated:
Foot launched ultralight gliders like the Swiftlight and the Archaeopteryx can fly almost everywhere in Europe, but for those which are not foot-launchable very few has been done:
Up to August 2018, the mass limitation was 80kg empty weight, which made them not very attractive. The 250kg new rule from August 2018 will generate more interest for the ultralight gliders.
Germany already has its LTL-L rules, France is working on the subject.