If someone wants to learn science subjects in a foreign language what should they do if there is not much material available comparing the two languages?Especially if one is yet to enter a university but plans to move abroad.
My wife did not quite study sciences, but physiology is near enough to offer insight, I think.
Get the basics down first
You'll want a reasonable grasp of basics of the language first. Academic material tends to not be limited to simple sentence structures and the concepts can also be complicated; problems with the language will add to the cognitive load of mastering the science, and you want to minimize this.
So learn to read well (enough). Just general texts or whatever. Some grammar can also help; conditional sentences and such.
Use resources in other languages, too
Presumably you will go to a school or university or whatever, or maybe study from a book, in the foreign language. Read or listen about the same material in your stronger language(s), too. This will allow you to understand the subject matter better and also allow you to figure out the new language. Just reading relevant Wikipedia articles in your stronger language(s) might be a big help. And if your stronger language(s) is/are big enough, there might be video lessons online, too.
Pick up science books for children
You might go to the local library and pick up illustrated science books for children or teens. The idea is not so much that the content is new to you (I hope), but rather that you'll pick up the basic vocabulary everyone is assumed to already know. In case of mathematics, maybe instead find some kind of school books or school review stuff.
Maybe there are Anki decks or similar
You might be able to find or make an Anki(droid) deck or similar resource for the specialist vocabulary you really need. Or maybe there is a list of technical terminology with translations that you can make use of. Clozemaster might have some exercises related to your field. Duolingo might or might not have a bit of vocabulary.
In any case, some tool for picking up the specialised vocabulary might be a good idea. Just reading Wikipedia articles might be sufficient.
Try to learn the science (in any language) and the foreign language (in general). Supplement a bit by focusing on the science vocabulary in that language.