At the end of the ARW there was a movement to make german the official language.
Gaaaahh! Not this again!
No, that is an urban legend, with some very, very little basis in fact. There were a couple of moves to allow German to have some official status (not THE official status) in Pennsylvania (not anywhere else).
Still. German as an active language in Pennsylvania, Dutch in upstate New York, French in Louisiana and *Acadia (if the US took New Brunswick, somehow), are all theoretical possibilities.
You'd probably have to cut off Anglo immigration before the mad rush in the 1800s, and I'm not sure how you'd do that. Either that or have even MORE immigration, and I don't know how to do that, either.