International applicants who have not acquired their study qualification at a German-speaking institution must prove German language skills that enable them to study at a university ("Sprachliche Studierfähigkeit"). Further information can be found here.
Exceptions are the degree programs M.Sc. in Economics and Finance, M.Sc. in Economics, M.Sc. in European Economics and M.Sc. in Management & Economics, in which you can prove your knowledge of English instead.