Got this sentence right without peeking. The logic behind this and many other words is often obvious - English needs to regain it in a way-
English does do it, more than we realize, we just tend to write the words as two words instead of one. Twitter post, car accident, desert varnish White House, bird house (as opposed to white house) - all have only one main stress, just as if they were a single word. We also have words like doghouse. and bilingual is two tongued, it's just borrowed from Greek (and Latin?)
It's exactly the same logic in English. "Bi" means "two", and "lingua" means "language". Half our vocabulary is Greek/Latin/Romance, and that is the resource we use for coining new such words. "Bilingual" is as transparent to an educated English speaker as "tvåspråkig" is in Swedish.
Almost. The last letter in "fyra" tends to fall away when used in compound words, so it's "fyrspråkig". It can even fall away when the number's just used on its own sometimes, like for example when you're keeping a rhythm ("One, two, three, four! One, two, three, four!" "En, två, tre, fyr! En, två, tre, fyr!"). The same thing happens with åtta (eight) turning into "ått". I'd guess it's to make it monosyllabic like all the other Swedish letters up to ten. This change doesn't happen in writing though.