Language requirements at German Universities
The official language in Germany is obviously German. And most university courses are taught in it as well. There are exceptions, though (there's even a catalogue of university courses in English).
Nevertheless, most universities have set language requirements for their students.
You need to be able to officially prove your German (or English) language proficiency to your university.
Always make sure you know what level of German is expected from you at your chosen university and your study program - sometimes different programs have different requirements for German levels. This information is always available on the university's website, mostly under applications section.