Writing not in your native tongue?

I'm not a native English speaker. It hasn't stopped me from getting small press contracts and a lot of full requests from agents. I doubt any of them can tell.

It takes several years to get fluent enough, but if anyone wants to make the effort, it's entirely possible.

It's not stopped me. Okay, so I make some mistakes, and sometimes I really have to stop and think about what I want to say, and other times I make mistakes without even realising they are mistakes, but I believe people understand what I'm trying to say, more or less, so it's all good

But to answer your question, I suppose it could work if the American did his/her research on Germany, or better still, went to Germany for some time.