UGH I HATE IT WHEN PEOPLE SAY THAT!!!! "If America had never joined the war, they'd all be speaking German."
For all we know, if America hadn't joined the war, Jesus Christ would have be born in Bavaria, converted Hitler to Judaism, and we'd all be speaking Latin or Yiddish or something. In fact, in Dick's the Man in the High Castle, in which the Axis wins the war, West coast America speaks Japanese.
War is a game of chance, and you can try to predict it, but most likely it's just going to spit in your face and laugh. The only history we have are memories and thing's written. That only gives us point of view and some facts. Why did America REALLY join the war? I doubt even the Congress that made the decision truly knew all the cards they were dealt, regardless of what they thought they knew.