I guess German is a must... Ich glaube das Sie mussen auf Deutsch sprechen!
A guy that I know went to Austria for an interview almost 1.5 years ago, unfortunatelly, he found out that German language was a must.
Find out all the requirements before going there for your interview... this buddy spent a month studying hard and lost some working days, plus money...
Good Luck if you go there, looks like a nice company and a nice place to fly...
AirGus