I'm not normally one to recommend UA anything to anyone, but this is probably the one case where it makes much more sense for you to go with UA Mileage Plus.

As a German resident, you will not be subject to the new spending requirement. And every Lufthansa segment in M, H, Q, V, G, U, W, S (excludes T and L) counts as 1 Premier Qualifying Segment (PQS).

You need 60 PQS in one calendar year in order to qualify for Premier Gold which is Star Gold, and gives a much wider range of benefits on Lufthansa than FTL, as well as all other Star airlines.

Summary: If you will be doing more than 60 segments in non-T/L class within Europe this year, start building miles on UA to achieve Star Alliance Gold and get a much better set of benefits.