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.