Nobody actually speaks “Austrian”. The language of Austria (Oesterreich, Eastern Realm) is actually a rather accented and stilted form of German, full of odd constructs and very formal greetings.
The former conglomerate empire of Austria-Hungary was actually a polyglot of literally dozens of languages, and Vienna was a melting-pot on the order of Paris. What was once part of Yugoslavia was carved out of the break-up of that empire after the members of the Habsburg family were deposed as royalty.
Judging from the family name, these girls were from perhaps Serbia, or possibly Bosnia, and at least knew about the Muslim culture of Kosovo, over which there had been a great deal of strife for centuries. That they chose to be gun molls running with Islamic outlaws, well, we have examples of young girls running off with motorcycle gangs in this country, as well.
Youthful rebellion is so, well, messy.
Our President, when in Austria, apologized for not being able to speak any ‘Austrian’.
The poster was making a joke, a very clever, albeit subtle one, at that!
There were lots of Yugoslav immigrants in Austria even when Tito was alive. I was in Vienna in 1979 and remember seeing an anti-Yugoslav graffito in a Catholic church (the Votivkirche) to the effect of "go home, scum, you aren't wanted here." That would probably have been directed at Croatian or Slovene guest-workers rather than Muslims (who wouldn't be likely to visit a church). I figured it had to do with German anti-Slav attitudes or just general anti-foreigner sentiments.