The rule
A spouse who wants to join a partner in Germany must be able to communicate at least in a simple way in German before entering. Both spouses must also be at least 18.
§ 30 Abs. 1 S. 1 Nr. 1–2 AufenthGThe law calls this “simple German” and defines it as level A1 of the Common European Framework of Reference for Languages.
§ 2 Abs. 9 AufenthG