Why does Karl have an American accent instead of an English or German one?

This has been puzzling me basically since I started playing the games, Karl is a native German, but also he spends most of his time working for the SOE suggesting he's probably a citizen of the UK and is part of the larger British military during WW2 and only occasionally seems to work directly alongside the US military.

Is it just a game thing that he doesn't speak in an English or German accent or is there an actual explanation behind it?