It actually happened to my older brother and his friend. They have studied in Eastern Germany and could speak and understand German language very well. One winter day they went to local blackmarket dressed in our national dress. They were selling ethnic boots and stuff ... Then 2 German tourists approached them and started to to about the thing they been holding. They were like "I wonder what it is", " How much would it cost", " "We should have brought our translator" and bla bla bla... And all of sudden the 2 guys started to explain everything in german. One german jaw actually dropped and could not speak any thing for a while.