While it’s plausible, it’s enough of a stretch that an explanation feels warranted. Being an outpost one would expect some level of offworld visitation, but Wookiees have never been shown to travel extensively. Chewie is virtually the only one ever seen outside of Kashyyyk or in slavery. Wookiees visiting often enough to learn the language seems very unlikely. However, it’s not hard to imagine a plausible explanation, e.g. there is or was a Wookiee scavenger on Jakku that Rey had been familiar with or she scavenged a language teaching device and taught herself several languages in her alone time at night. Given how few people seem to speak Shirywook, it feels like there should be something given on film.