How a word is affected by grammatical rules (such as pluralization) is not determined by etymology (but it can be).

Here's the german conjugation of "mailen" (writing an e-mail), borrowed from "to mail":

Ich maile, du mailst, er/sie/es mailt, wir mailen, sie mailen.

I don't know any loanwords that break english pluralization rules in german, but for the reverse: The correct plural for "Kindergarten" would be "Kindergärten" (not "kindergartens"), which I imagine some english speakers would have problems with. And "Autobahnen" is rather unintuitive compared to "autobahns".