I think the US should teach a foreign language in Elementary school. At minimum some sort of exposure, even if just twice a week.
As far as whether to teach other subjects in a foreign language, I think for most students it’s good. I think total immersion in a foreign language in elementary school is great for many children. However, for some kids it is detrimental and we’d have to account for that.
Hell, I think the new way we teach math with so many wordy math problems from a very young age is a problem. Kids who would be great in math, but have difficulty reading now can’t do math well either. I think very young children should have the disciplines more separated, but the trend is going the opposite direction.
Also, children who move a lot might have trouble. I woman I worked with learned history and geography in Spanish and to this day she knows country names and some historical figures by their Spanish names better than their English names. That’s not a huge deal, but could affect a point or two on a test score.
Too bad two to three generations ago immigrants though it better for their children not to be bilingual. They wanted their children to fit in, be American, and not have an accent. Now we know people can be bilingual from the get go and switch accents and move between languages with no significant problem. Imagine if that generation had maintained their languages in their homes. America would have many many more bilingual and multilingual citizens.
I know I don’t want two official languages like Canada. I also know I want children to be bilingual and I don’t want to mandate what language besides English will be taught.
Another negative is that at young ages bilingual children do tend go get their grammar more messed up than their single language peers. It eventually sorts itself out. By age 5 or 6 it already is fairly sorted.