As an aviation English teacher I am interested to see how quickly the language changed to Turkish - in fact it was ATC who initiated it. It is obiously more efficient for the two native Turkish speakers to revert to their own language in such a stressful situation, but I wonder how efficiently non-native speakers (especially different first languages) would have handled the situation. I have an English speaking friend who flies for Turkish Airlines - he would have been a bit flummoxed if he was sitting in the right hand seat and ATC switched to Turkish!