In accredited universities and college, lectures are principally held in German.  There are a few exceptions, when the lectures are held in English. An overview can be found here >>> 

International students studying in Austria must prove adequate German language skills. For graduate level studies, the language requirement is level B2 and in some cases level C1 (European language leves (CEFR).  There are other qualifications that may be recognized, but for this information it is important to find out the details directly from the university.

University preparation programs  in Vienna, Graz and Leoben prepare students for the targeted study programs. For such studies there are university entrance qualification, particularly for the subjects German, English, mathematics, physics, chemistry, biology, history and geography.

Companies such as the Österreich Institute provide German language courses abroad with partner companies.

Also, there are many language schools in Austria to prepare student to successfully complete the German language exam.

The "Goethe-Institut" of Germany offers language courses in 92 countries.

Information about German course providers, test dates, and opportunities to learn German on your own

The Austrian Integration Fund language portal