This one is pretty well-known: the notation $e$ for the identity of a group comes from the German word Einheit, meaning unit.
I'd be willing to bet that the notation $G$ for a group also comes from German... but we don't notice, because the German word for group is Gruppe!
Here's a fun one: the notation $Z$ for a topological quantum field theory comes indirectly from the notation $Z$ for a partition function in statistical mechanics, which comes from the German word Zustandssumme, meaning state sum. I said "indirectly" because partition function in quantum field theory isn't a statistical-mechanical partition function... it just looks like one after you Wick rotate! (Then again, maybe there's a deeper sense in which the QFT partition function really is a statistical-mechanical partition function. Does anybody know?)