The German for adder is Natter. *smartass*
I think there are a number of English words that lost their initial an. Because people started to feel that "a nadder" should be "an adder". Or so I heard, I wouldn't put my hand into the fire for that. Might have been on that BBC show, so it would probably be true.

A new subject I want to to talk about: I was just playing Skyrim and searching for an arrow I accidentally shot in the ground, and always watching and playing everything in English or Japanese, my talking to myself was in English and so said "Ah, there he is." Which for an arrow in German makes sense, but is nonsense in English.
So I was wondering and asking those who speak English well as a second language. What mistakes are there that native speakers of your languages often make when they are speaking a kind of broken English?

Assigning gramatical genders to objects would be one that Germans probably do a lot. In English it's only done with ships or rarely countries. (Interestingly, in German specific ships are also feminine, while neither the words ship or boat are. Probably a tradition that doesn't have anything to do with grammar.)

One my mother told me is that Finns tend to mix up he and she for people, apparently because Finnish doesn't have that distinction. She's something like on the pannell for an annual baltic film festical and said she met a number of Finns who made the mistake.