It turns out that C-3PO made good use of his time between Star Wars: Return of the Jedi and Star Wars: The Force Awakens by learning a few new languages.
Though his part in Star Wars: The Force Awakens was relatively small, it was clear some changes had been made to his character.
“By the time of The Force Awakens, C-3PO has upgraded his programming to include over 7 million forms of communication.”
Hopefully we will get to see him show off some of these new languages he learned in Star Wars: The Last Jedi or Star Wars Episode IX.