I guess we see her speak a couple other languages there with other people at or around the outpost.
We do see Rey communicate with Teedo; it’s clear Rey knows who he is, as in her first scene with BB-8 she tells it “That’s just Teedo.” But there is no indication he’s from a different planet, so we can conclude from what we know that he is from Jakku.
Her having skills learned offscreen isn’t being a Mary Sue
I understand that, but if it contradicts her background presented to us as well as established in-universe facts, then it would make her a Mary Sue. For example, my character is a regular human baby who lives in Hell and speaks normal English, after being abandoned by his English-speaking parents moments after it was born; in Hell the only language everyone ever speaks is perfect and normal English. But somehow he understands the Navajo language, literally on the verge of “extinction”; this would make my character a Mary Sue/Gary Stu.
Shyriiwook must be a very straightforward language, since even 22-year old ex-scumrat and Imperial Academy expellee Han Solo seemed to be able to understand it.
If I recall correctly it’s established in Solo the Empire is enslaving Wookiees, so I don’t think it’d be hard to conclude Han learned what the language is specifically during his time at the Imperial Academy.