The "we" in the question/premise is not homogeneous. E.g., mathematicians of my age or even a bit younger in the U.S. would have been required to demonstrate some nominal reading proficiency in French and German, with Russian as a possible alternative. Latin, French, and German were taught in most high schools in the U.S. then, as well as Spanish, and it was no secret that studying Latin, French, and German was a good idea for anyone interested in the sciences, or possibly philosophy, and other things that were valued in the high-culture of Western Europe in the late 19th and early 20th centuries.

Indeed, "when I was a kid", many of the primary sources were not in English, and no one thought to object, or even remark upon it.

What "we" in those times often did "miss" were developments in Russia, written in Russian. Most Japanese writers pre-WWII wrote in some European langage, German, French, or English.

Many Russians (pre-revolution) wrote in French or German.

Quite a few people from the U.S. wrote in French or German in those times.

Nowadays, for many things there are indeed English-language versions. But this is not absolutely true, and it is invariably convenient to be able to read some French or German. But also possible to not do so...

What are "we" missing? Well, probably not much so far, because there is still an older generation familiar with languages other than English... but there indeed may be something forgotten a bit later...