What makes these two words so different that 'man' is changed to 'men', but 'German' is changed to 'Germans'?
German is from the Latin word germani; unlike the similar-looking demonyms Englishman and Frenchman, it is etymologically unrelated to the word man and does not form a plural the same way.
Another way of saying it- a man from France is called a Frenchman, and many of them are referred to collectively as FrenchMEN. A man from Germany (we see here with the "y" that "man" is included as a coincidence- you can't take it as "many from Ger" either...) would be called a Germanman and many would be Germanmen. I think they might have said it back in WWII, but I could be wrong.