My head-canon is that the reason no one ever translates for R2 is that he's extremely insulting in virtually every sentence, not just to 3PO, but to everyone.
There are lines where 3PO is like "I can't translate that" so your theory is proven to be right
My favorite R2D2 line is before they meet Obi Wan in New Hope and he's all like "ooooooo OOOOOO" in a frightened tone.
In the early scripts, R2D2’s lines are… quite interesting.
Fucking hell, he’s a massive fucking nerd.
I knew I liked him. he also has some fairly perverted comments. usually those got replaced by fart-noises.
Wait, not everyone does this? I thought everyone knew R2 was a foul mouthed manipulative little shit.
My head-canon is that the reason no one ever translates for R2 is that he's extremely insulting in virtually every sentence, not just to 3PO, but to everyone.
There are lines where 3PO is like "I can't translate that" so your theory is proven to be right