I think it's really polite of C-3PO to talk to other droids and things in human-language. Higher throughput, lower error protocols must exist, but he totally doesn't want the humans to feel left out of the conversation!
R2D2, though, is like one of those really stand-offish people who deliberately talks to their friend in French or whatever whilst the rest of the group just stands around awkwardly.
Then C-3PO replies in human-language, to make a point. You're all about the social glue, C-3PO.