While the force may be strong with him, I still don’t see how Luke Skywalker could understand a single thing R2D2 was saying. But somehow they managed to not only carry on many conversations, but provide us with enough expository dialogue so we could follow along too.
After 30 years from when Star Wars was first released, it's great to see someone finally decipher the R2-unit dialect, and provide us with a handy online translator. It is not recommended using headphones to listen to the translations because some of them are particularly high-pitched and shrill. But once your phrase is translated you do have the option to download the sound file as an MP3 and use it as a cellphone ringtone or an annoying voicemail message.
Source: R2D2 Translator