To get a German citizenship, you either have to have lived there legally for 7 years and fulfill some criteria (language, no crime, job etc.) or have German ancestors. Unless you are European, you also have to prove, that you gave up your old citizenship.

Fifa also has rules preventing this. You cannot switch national teams, once you played for one.

It is actually the other way round: After the poor performances in the early 2000s, German clubs have agreed on a few rules regarding how many foreign players are allowed to play on each team and what each club has to do to train new young players. The richer clubs basically restricted what their own money could buy. These regulations are widely regarded as the reason for the dominant years during the 2010s.