German law forbids to show propaganda material of banned parties. This includes portraits of Hitler as well as SS and SA symbols, swastikas, the hitler salute and a whole lot of other stuff.
However, all this is allowed to be shown if "the propaganda material or the act is meant to serve civil education, to avert unconstitutional movements, to promote art or science, research or teaching, the reporting about current or historical events or similar purposes." - StGB 86 (3)
So I personally think this exceptions are sufficient for his portrait to show up in a game about history, but I'm not a lawyer (and I hope Paradox asked one). Hitlers face shows up in every newspaper like once a week (and in certain "history" or "science" publications more like every day) so I can't imagine this would really be a problem.
It is funny how the german laws are trying to keep citizens away from national socialism. "Hide Hitler's face and let's pretend nothing happend 80 years ago"
Everyone going to a german school has extensive lessons about the Third Reich, like in total a third of all history class sessions. I personally had subjects related to Nazi Germany in grades 6, 8, 10, 11 and 12, up to the point where it becomes annoying if you hear the exact same things again every few months.
And if you pretend nothing happened 80 years ago in Germany you will be trialed for that.