As an international couple, we’ve had many misunderstandings – most of them are funny like this one, though some have been a lot less funny.
In England, you cross your fingers to wish someone good luck. “I’m crossing my fingers for you!”. However, in Germany, if you say something and cross your fingers at the same time (the gesture), you are saying that you are lying, or don’t mean to stand by a promise you are making. For example, a German child would promise not to eat the chocolate, and cross their fingers behind their back (because they don’t mean it).
Instead, Germans make a fist with their thumb inside, called “pressing your thumb” (Daumen drücken). When a German seems to violently shake their fist at you with a smile while shouting “Viel Glück!”, they are indeed wishing you good luck. Germans be warned, Brits might not immediately understand this to be a friendly gesture… at least my husband thought it was rather comical when little me shook her fist at him.
So, when you wish someone good luck in Germany and show them that you’re crossing your fingers, don’t be surprised to get shocked or rather angry looks in return. You’ve just told them “Good luck! Haha not really”.