Anakin was able to understand it. Luke seemed to at least a little, at least when R2 was plugged into his X-Wing, though that could have been due to some unseen translator in his helmet or something.
There is a deleted scene from ROTS wherein it shows that the little communication device Anakin uses to call R2 actually has a translate function. This is on Dooku's ship. When it temporarily malfunctions, there is a brief comedic bit where Anakin & ObiWan argue over what they think he's saying, which implies they kind of understand him, but not fluently.
It's on YouTube.
Maybe not technically canon, but I think it shows the intent.