Yes. In the original trilogy, he does his job in each of the three movies.
- Translating R2D2's beeping-whistle language for Luke.
- Specifically, translating R2's warning about the approaching life forms (Sand People) on Tattooine.
- And most importantly, telling Luke that R2 has located Princess Leia aboard the Death Star.
- Speaking with the Millenium Falcon to help diagnose its problems
while in the asteroid field.
- Translating the language of love between Han and Leia, and interceding at the most inappropriate time for the two of them.
This is where C3P0 really gets to shine.
- Translating for Jabba the Hutt, which while not exactly heroic, IS
his job as a translator.
- Translating the Ewok language for the
rebel strike team.
- Most importantly, translating the strike team
for the Ewoks, and convincing them that he is a Golden God of great
power and anger.
I can't recall him doing any sort of translating in the prequel trilogy though, not even for R2. Though presumably he may have, at some point, acted as translator for Owen Larrs. And at the beginning of Episode 3, he's clearly working for Padme in some regard, though we never see any of it on-screen.