I’m working on a project for conlanging, where you’ll be able to build a dictionary with as many information as possible (etymology, declensions, tenses, etc). The first thing I’m gonna do when it’s finished, I’m going to build a dictionary for my native language, Slovak. It will show how many borrowed words it has.
We have a lot of borrowings of Greek and Latin origin, for which we also have Slovak synonyms (agrikultúra – poľnohospodárstvo; helikoptéra – vrtuľník; edukácia – vzdelávanie; etc). I think we can ignore these words.
There are some Slavic words I wish we had in our vocabulary, like kolo instead of bicykel, samochod instead of auto/automobil, dŕžava instead of štát and so on…
I studied Russian and Polish for a very short time, though I found them to be infested with many foreign words even for basic stuff like food, clothes, travel, etc. However, I don’t know the percentage of foreign words for the languages.
I don’t know the answer for your question and I doubt anyone has ever tried to do studies on this topic. It’s impossible to answer right now. We can only theorize.