As was already mentioned, the strobing and flashing is not according to the StVZO. And as you also have realized correctly, nevertheless, Germans are using such lights. In the German part of this forum, recently, there was a discussion about this. Some of our forum members find the flashing annoying. Some others - like me - do not really care. I myself use a "regular" rear light according to StVZO. Usually, police will not complain about a flashing light, since they are in most cases happy to see a light at all . More complicated would the situation be in case of accident during night time. Insurances will try to fight claims by any means and if lights were not according to rules, there might at least a reduction of compensation payments be enforced.

Buying a head or rear light in a German bicycle shop will in practically all cases mean that you will receive a light according StVZO. EBay or other internet recources will also easily give access to Chinese no-name brands, which do not have the required approval mark (waveline with K + number). This is against the law if the seller is not private. Police nevertheless officially can tolerate such lights.

A good overview is given in Wikipedia, however, for German regulations of course in German (see HERE)).