So much this. Rocky's weird language quirks are because he's simplifying and restructuring his language to make it easy for Grace and/or the computer to pick up.
Rocky is doing the hard translation work in his head before he speaks because Grace is learning and the computer can only regurgitate words that have already been saved — in the order that Rocky says them.
Like when you have a friend who is learning your language, and you slow down and use simple words/sentence structure and fewer slang/jargon words to make it easier for them.
Because hearing is Eridians' primary sense, they have a better memory for sounds, which means Rocky picks up English quicker than Grace can pick up Eridian.
In the book, there is no text-to-speech involved, Grace records Rocky's vocab into a spreadsheet, which he uses for reference on the fly and also as a DIY duolingo for Eridian practice. The movie just adds a speech generator (which is a pre-recorded soundbank provided by a NASA journalist, played by James Ortiz, lol).
But it's doing the same thing — just out loud for the audience's benefit (and so Grace doesn't have to be running back and forth to his spreadsheet in the early days). It matches soundwaves that Rocky generates to the vocabulary spreadsheet and then pulls the corresponding English word from the soundbank.
Obviously it *should* sound very early GPS voice 🤣 because it's word-by-word without infection, but although Ortiz did try that a bit, it wasn't working well on-camera. So they turned him loose to give Rocky's voice personality. And we can suspend our disbelief for it because he's *fantastic*.
But especially when writing for Rocky in fics, we need to remember exactly what's happening, otherwise it won't sound authentic.
Mary's voice is likewise not really ai. It's a voice assistant with a prerecorded soundbank as well. It can really only respond as programmed.