Just to add: Under German law even crowd pictures are technically protected under the same law which means if you take a picture of a crowd of people you theoretically would need each and every one of these people to give you a permission to use their picture in writting. For that reason German concert tickets for example have a note on them that you agree to have your picture taken with purchase of the ticket and if the concert is really recorded for commercial use there will also be signs at the event reminding you to leave the premises if you disagree with having your picture taken. So the flight attendant was definitely in the right here.
But much more importantly than this German law thing is that taking a picture of somebody without their permission can get you in really big trouble. In some parts of this world taking a persons picture equalls "taking their soul" due to religious believes. Therefore you should definitely always ask for permission before you take a picture of a stranger.