The air traffic controllers communicate with the pilots in German. In reality, all communication between the air traffic controllers and the pilots of commercial airliners must be in English, which is the international language of aviation.
The air traffic controller controlling the airspace at the time of the crash uses the call sign "DHX" when communicating with the DHL cargo plane. The cargo plane involved in the real crash was operated by DHL International Aviation ME (SNAS/DHL), which uses the call sign "DILMUN".