Star Wars: The Rise of Skywalker saw C-3PO going through a memory wipe so the group could retrieve a translation from Sith language – but how did C-3PO know that language in the first place? C-3PO is one of the few characters that made it to every movie in the Skywalker saga, meaning he witnessed a lot of battles and conflicts, but he also made some really good friends along the way. Although he’s a very popular character, not only within the Star Wars fandom but also among those who are not into it, C-3PO was a secondary character through most movies, but he had a big moment in the sequel trilogy.
Along with R2-D2 and Chewbacca, C-3PO is one of the characters from the original trilogy that was brought back for the sequels and didn’t die, and he was key in the plot of Star Wars: The Rise of Skywalker. When Rey, Finn, Poe, Chewbacca, BB-8, and C-3PO travel to Pasaana, they find a dagger inscribed with Sith text, which none of them can read – except C-3PO. However, his programming prevented him from revealing the contents of the text, and so they travel to Kijimi so droidsmith Babu Frik can lift the block. Once there, Babu Frik tells them that doing so would cause C-3PO’s memory to be wiped, and so the loyal droid decides to sacrifice himself so he can help his friends.
This hidden ability of C-3PO raised a lot of questions, mainly how he even knows Sith language. C-3PO is a protocol droid, and as such his job is to aid sentients with etiquette and relations, often dealing with translation, problem-solving, event planning, and more. C3-PO is a special model within his type, as he was built from spare parts by a young Anakin Skywalker (as seen in Star Wars: Episode I - The Phantom Menace). During his time in Tatooine, he served Anakin and his mother, and later helped Anakin make some adjustments to his podracer. At the time, the Sith were believed to be nothing more than a legend, so it wouldn’t make sense for Anakin to add an obscure and rare Sith language to C-3PO’s protocols – but what’s possible is that some of those spare parts came with some old Sith knowledge.
Years later, C-3PO was gifted to Padmé Amidala by Anakin Skywalker, and began his service as a proper protocol droid, in charge of communication and translation. By then, the Jedi Council had started to suspect the Sith were rising again, so it wouldn’t be outside the realm of possibility that C-3PO got some new abilities during his time under Padmé’s service, though that would make the question of why he had a block installed, preventing him from sharing what the Sith texts said, even more complicated. It’s also possible that Sith language was simply something standard in a droid’s protocol, but then the block wouldn’t make much sense again.
Although the Skywalker saga has now come to an end, this part of C-3PO’s history could be solved in novels and other (canon) media, though it’s also likely that it will continue to be a mystery. The Star Wars sequels clearly didn’t follow a plan, which makes seemingly unimportant details like C-3PO knowing Sith language very confusing and, to a point, non-sensical. But for now, the possibilities could be that it was something that came in one of the parts used to build him or was added when he started serving Padmé.