Is knowledge of the German language mandatory for studying in the program?

Yes, if you apply for admission, you have to a approve that your level of German is at least B1. Which documents are valid, can be found under “Spezielle Kenntnisse 2” here https://gremien.hu-berlin.de/de/amb/2019/26/26_2019_zsp-hu_2013_ae07-2019_2019-05-01_druck.pdf#page=44

The mandatory course Stochastik I is in German but almost all others courses are held in English.