Maybe understanding droid is something the Jedi teach, along with the other skills. Poe would have learned it growing up with a pilot mom and when he trained it would probably come up. Rey might have learned it from Unkar Plot. Luke might not have had the time to learn it in the OT.
Regardless of who exactly Rey learned it from it is stated in the Visual Dictionary that she learned the language of the Wookies and Droid Binary during her time on Jakku. So in that department Rey is in the clear as far as that Mary Sue stuff goes.