Pit any character against a Nazi and the audience will automatically know who to root for. It’s much harder to show how a reasonable person can become a cold-hearted monster. The zeal with which teenagers approach any popular interest makes them ideal recruits; it only takes a charismatic mentor to offer all the things the War has taken away, which no amount of Big Jazz, Swing dancing, and jive slang could provide. Friendships are strained as loyalties are tested. Could anyone/thing escape the grasp of the Reich?
Incidentally, (and thankfully) no one attempts a fake accent. They all speak American or British English and you don’t even notice.