You could make arguments for her understanding droid not merely being a plot convenience but her understanding Wookiee certainly is.
She’s on a hot desert planet in the middle of nowhere. I doubt there’s a colony of Wookiees somewhere off screen on Jakku. There’s no reason whatsoever for her to have learned that.
I don’t really need to see Rey interacting with Wookies or even see one in the film to buy her being able to speak their language just like I don’t need to see Luke flying to buy him being able to fly in the Battle of Yavin. Just because STAR WARS has more lines in regards to Luke flying doesn’t dismiss the one line mentioned about Rey being able to understand that ‘thing’ in The Force Awakens.
In every iteration of the Star Wars galaxy that there’s been, Wookiees are a rare sight.
There are thousands of sentient species with millions of languages between them. Unless Rey’s last name turns out to be 3PO her knowing Shyriiwook is outlandish at best.
It’s like finding a girl raised in a junkyard in Manchester, England that can speak Karuk.