What would happen if some phone manufacturer produced an R2 phone that when you asked, “R2, where’s the nearest coffee shop?” it replied with a series of beeps and whistles in a language that you would need to learn in order to know you go two blocks and take a right? I’d bet it wouldn’t sell. A few nerds might buy one, but most people – if they really wanted to – would probably just download an app to the phone they already have. It would be cheaper and you could delete it once the novelty wears off. (I can picture someone yelling at their phone, “Is whistle left, or right?)
Given that C-3PO is “fluent in over six million forms of communication,” the language memory bank can’t be that big yet still fit inside his head. So why doesn’t R2-D2 have a similar memory bank? Or at least one that lets him have a second form of communication like basic English which most – if not all – beings in Star Wars understand.