They don't understand because the majority do not speak or understand English. And i'm pretty sure the offender was not German or English. We get this a lot, we have them arrested and they still don't get it...
If this happens mainly on the flights from Berlin, then it's very likely that they're Polish. (It's cheaper for them to drive from some parts of Poland to Berlin than to fly from their local airport). Perhaps a sign in Polish would help?
(And before everyone starts to tell me that they all speak English - not when they're in trouble they don't (personal experience!).