Rocky did most of the translation work
I've seen takes along the lines that the speed at which the characters are able to understand each other is unrealistic. Also, assertions that it's not realistic that Eridian would have English grammar.
Let's set aside that Weir didn't want to write 200 pages of linguistic reverse-engineering because he doesn't find it as interesting as he finds the more technical problem solving aspects of the story. I think people often miss the fact that Rocky is doing at least as much work as Grace to bridge the communication gap.
Rocky has better auditory processing and a much better memory than Grace. He learns English very quickly because his brain is much better at learning languages. Then once he's learned English, and how Grace prefers to communicate, I think he just proceed to talk in an English-grammar babytalk version of Eridian. In other words, he's tailoring the way he speaks to be easily translatable into something Grace can understand, leaving Grace's computer to simply translate word-to-word mappings.
It's a theme of the book that the Eridians are good at things humans are bad at and vice versa, and they can only save both planets by working together. Grace isn't suited to learning languages; therefor, Rocky is.
I feel that this reading is pretty well supported. Rocky does many helpful things to bridge the communication gap that go basically unremarked upon, or are only recognized in retrospect. For example, building the first window out of many materials to see which one Grace will prefer. This is a powerful signal of how much effort he is constantly putting into communication, which Grace and the reader to a large degree take for granted.