Sadly it didn't explain why no on had ever bothered to program R2D2 to speak a human language.
The basic premise of the Star Wars universe is that in order for you to communicate with a different species, you must understand its language, but not necessarily need to speak it, since your physiology might prevent it. So it kinda makes everyone a polyglot. It isn't any different with R2s, they are service androids and need to speak to machines, not humans.
3POs exist because the speaking apparatus for any given two races might be so different that an interpreter would make communication more fluid, and as a protocol droids they tend to filter out any rudeness in the speech.