Well, technically speaking, it is in his memory banks according to the movie. He just is prohibited from sharing any information obtained from the translations due to a restriction coded into him.
Which sorta makes sense based upon what Hal did for episode 1. Since Anakin didn’t build him now, but repair him, it’s possible he came preinstalled with Sith language. Who knows what he was used for before Anakin got him. The droid could be thousands of years old, who knows?