There are two distinct elements here.
Could a country have conquered the United States?
and
If a country conquered the United States, could it have changed the language?
Sub to that question is would any country want to change the national language?
As for the first question, there are all sorts of “what if” threads that come and go, and scenarios in which America gets its ass whumped aren’t that hard to come by. Forget hiney-whomping for a moment, just being roundly defeated in WWII or due to electoral differences (hey, who’s the asshole with the time machine and the animosity for butterflies?!) staying out of it could (note: could) have changed the outcome of the war.
I can’t imagine that there aren’t many very plausible, very well-researched works of fiction out there.
Now, take a weakened post-war US–one with no Manhattan project–and give the atom bomb to either Germany or the Soviet Union. Both seemed to have little reservations to killing a lot of people who got in their way. Give them ten years to rebuild (whichever country you want to imagine for the moment), a nuclear arsenal, and need/desire for whatever resources we have at the moment.
Why is it such a stretch that if not for a handful of lucky breaks and a lot of sweat the post-war period could have been much different. Perhaps I should post to the “things I was taught that were wrong” thread, but I thought victory in WWII was far from certain.
So is it that implausible that Russia/Germany could have emerged as the sole, dominant, and ambitious superpower after WWII? No.
Is it implausible that the US could have been severely weakened (or never militarized in the build-up to WWII) in the post-war period? no.
Is it implausible that a dispute, real or otherwise, could have sparked an aggressive action against us? No.
Is it implausible that after New York, Washington, Philly, and even Des Moins are irradiated (even with small nuclear bombs) the remaining power structure wouldn’t capitulate and surrender? Nyet.
Now what?
I went with the Soviet example because they had a pattern of pushing their language on “host” countries. Why? Stalin was like that. Or maybe there’s some other reason. But while they never completely eradicated all native languages, there are some that are lost, and there are tons of people who do not speak (or are relatively just learning to speak) their national language. Again, I doubt they’d have been able to jam the metric system down our throats, but removing the military and playing “what if” games only requires a modicum of imagination to get to that result.