Okay. Star Wars. In the Nineties, after I'd spent years watching the films and listening to the John Williams scores (and even occasionally watching the animated shows Ewoks and Droids), it finally dawned on me to ask: How do people understand R2-D2?
More to the point, how can he be understood? When he speaks, he uses a relatively limited range of electronic beeps, boops, and squawks, which sure doesn't sound like a wide-ranging vocabulary. You can get tone from him, whether it's "concerned," "amused," "in pain," or more, but it seems like you'd need more cues and clues for nuance. When Luke Skywalker is flying in his X-Wing with R2, a computer translates R2's speech into the written form of Galactic Basic, or "Aurebesh," but otherwise detailed conversations only happen when C-3PO is around.
So my theory was, R2 makes sounds we humans can't hear. Plenty of sounds are like that! That way, 3PO and the local equivalent of dogs hear more from him, and 3PO gives the humans the full, nuanced message.
I like my idea. It's almost certainly not what anybody connected to Star Wars considered when inventing R2-D2's way of talking, but it doesn't have to be! It's an amusing-to-me way to explain it.
Years after I thought that, the Star Wars prequels started coming out. (Standard-for-me disclaimer: my feelings about the prequels will always be more complicated than my feelings about the original trilogy, but I do like them. At worst I'm still mixed-to-positive about Episode I: the Phantom Menace. And yes, I honestly like Episode II: Attack of the Clones.) Smartly, when a younger R2 is introduced, he speaks with the tones we'd heard before, plus more variations on them. More vocabulary is implied! And the prequels also show that humans can understand R2 and other droids to an extent even without translators. Then there was the nice, unexpected joke in Episode VIII: the Last Jedi where R2 sees Luke for the first time in years and beeps really excitedly, and Luke replies "Watch the language." Heh.
Ultimately, "how are droids understood?" is what's called a "refrigerator question": you may only think of it after seeing the movie, getting home, deciding to eat, going to the fridge, opening it then going "Hey, wait a minute..." So, no big thing.
But you know what's still unexplained? How in Citizen Kane anyone knows Kane's dying word when no one was in the room with him when he died...