In the US, with the constitution, the right to free speech, no matter the language, would protect them. I think the Supreme Court might sway towards the Constitutional. I don't know where the issue of productivity comes in if they speak their own langauge. If anything, I'd think productivity would be fine or better if they were left to speak their own language. It potentially means getting new markets that couldn't have been reached before.

The collegiality issue might be true but even if they spoke english, I wonder how much they'd be ostracized because they didn't speak english right, didn't wear the right clothes, etc.

Last time I checked, I don't think the US had an official language like in Canada. We have written into our constitution that there are two official languages: english and french. Language laws are critical here. Perhaps you might one day end up with "language police" like they have in Quebec today.