German here. Am I tired of the portrayal of my country? Not really, but...
Well, I have no problem with any games where the Nazis are evil, because they were. That is not misrepresentation. I understand why they pop up so much in media (clear and easily identifiable enemy), and I have no problem with historic or Sci-Fi settings with Nazis in them.
What I have a slight problem with is the connotation of Germany and Nazis without context that is sometimes found in media. This normally manifests as german = evil in some subtle or nor so subtle ways. In most Hollywood flicks, the bad guys drive german cars, for example. And while that is not particulary harmful (really, it is rather amusing when you start noticing it), there are worse cases out there. Like the ONE person in the whole Star Wars galaxy I ever heard speaking with a german accent: an evil scientist who delevolped a deadly virus, not out of necessity of war, but because he liked doing so appearantly. In a show marketed towards kids. Developing and using chemical weapons wasn't even a german thing historically, the japanese did that! ...Sorry, pet peeve.
Interestingly, another example comes from the japanese. In the Senran Kagura games, one of the characters has german names for her special move weapons. One of these is called "Wolfsschanze". This was the mountain fortress Hitler and his generals planned most of the second world war in once it had started. I get that this was probably not intentional and they just wanted cool sounding names, but that is another problem in and of itself: Don't use a word if you don't really now what it means. ...Sorry again, pet peeve number two. But yeah, I found this one instance in the game problematic, not the fact that you can dress various "underage" girls down to their underwear.
So in short, saying that the germans were Nazis once is not a big deal. Connotating Germany with Nazis even today, especially without context, rustles my jimmies.
The thing that would make America a great villain in a game is that not only would an invading american army pose an immediate threat, it would be believable. Maybe you remember some of the stunts CoD and Battlefield pulled to make America a dangered and wounded country in their games. None of that here. America invades, boom, you have a mighty big problem, no matter where on the globe you are. America would be another easy villain if it was used, because just like the Nazis, everyone can tell you why America would be dangerous and had to be stopped. They spend more money on their military than some other countries make, so of course they are going to be a threat.