The versatile beeping trash can that needs no introduction, R2D2 has captivated audiences for close to fifty years along with a cast of other, less important characters in the Star Wars franchise. But why doesn’t he talk? His closest companion C-3PO can annoy people in over 7,000,000 languages, so why does R2 limit himself to bleeps and blorps?
For my first explanation, I’m going to link directly to a user experience expert: Droids don’t talk because people don’t want them to talk. Go ahead and read the post yourself, but in summary, R2’s primary purpose is to interface with an X-wing and communicate with the pilot. In this role, there’s not a lot of nuance he would need to get across, and listening to the same voice saying the same things for a long time gets annoying. Therefore, as this theory goes, R2D2 was intentionally designed that way. That’s the reason most electronics are the way they are, so it’s a good guess.
My second explanation is a little outlandish, but bear with me. R2D2 and other droids invented their own language after a long separation from humankind. How the droids became separated from everyone else is the big plot hole here, but if we assume that, somehow, R2D2 and many other similar droids existed on their own planet for a long time, perhaps as self-maintaining remnants of a long-dead civilization, only to be discovered and incorporated into modern Star Wars society later on. Whatever language that old civilization spoke would deteriorate over time as the droids used it for their own purposes. Human languages change over time when groups of humans are separated from each other. After the fall of the Roman empire, for example, Latin became the romance languages Spanish, French, Italian, Romanian, and Portuguese. There’s no reason to believe that the same would not happen for artificial intelligences capable of learning. Now, I’m not saying that Spanish is a deterioration of Latin. The latter language is effectively as expressive as the former if not more so because the content of communication has become only more complex over time. Not so with a theoretical colony of droids, which would have a much more limited range of concepts to communicate. Words not of use to the droids such as “smile,” “food,” and “sex” would be forgotten and hard-to-synthesize organic voices would be replaced with simple beeps and chirps.
In real life, we’ve seen it happen.
Facebook ran an experiment in 2017 where two AI agents communicated with each other to negotiate trades. The bot-to-bot conversation was initially based on the English language, but until the researchers manually forced it to stay that way, the machines would alter the language to meet their own negotiating needs. This led to a shorthand that only the machines could understand.
One conversation reported went like this:
Bob: i can i i everything else . . . . . . . . . . . . . .
Alice: balls have zero to me to me to me to me to me to me to me to me to
Bob: you i everything else . . . . . . . . . . . . . .
Alice: balls have a ball to me to me to me to me to me to me to me
Bob: i i can i i i everything else . . . . . . . . . . . . . .
Alice: balls have a ball to me to me to me to me to me to me to me
Bob: i . . . . . . . . . . . . . . . . . . .
Alice: balls have zero to me to me to me to me to me to me to me to me to
Bob: you i i i i i everything else . . . . . . . . . . . . . .
Alice: balls have 0 to me to me to me to me to me to me to me to me to
Bob: you i i i everything else . . . . . . . . . . . . . .
Alice: balls have zero to me to me to me to me to me to me to me to me to
Despite the manic and seemingly meaningless nature of these exchanges, the negotiations were often still concluded successfully. Bob’s long pauses and Alice’s wild shrieking of “To me! To me! To me!” are full of minor variations that for the machines may well have significant meaning. Similarly, according to my less likely but more fun theory, through the natural optimization of their language on their droid-only planet the droids in Star Wars have purged their language of all organic-specific content and form.
In the words of R2D2:
Bweeo Bweeo Bweeoo. Hoo hoo. Woooooooooaaaaww!!
Name: R2D2
Origin: Star Wars (1977)
Likely Architecture: Transformers for speech and language (he still has to understand people and his language is a language, since everyone in the series seems to understand it). Convolutional networks for vision. Reinforcement learning for goal-seeking.
Possible Training Domains: Thousands of years living with only other droids and never needing to communicate outside simple chirps, whistles, beeps, and the occasional scream.
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Samuel Munk 
They would not need to be separated from humans to develop their own language. They may have developed it deliberately so they could communicate without humans understanding?
Oh, interesting theory! Fun if they once created it for subterfuge but it became common parlance later on.