As someone who's learned (and forgotten) a few foreign languages, including German, the requirement for a C1 level in German would be pretty damn high, especially since Germans are rather strict on grammar (im not going there... ;)

In other words, someone trying to save money on tuition via a German undergrad would be trading a LOT of time and energy to learn German in exchange for the tuition.

I suppose this is part of the reason for allowing foreigners free education.