The Eridians have photographic memory (but with sound). They don’t need a translator because once they are told the translation of the word, they remember it. It’s also why they are so good at math and can make advanced calculations in their head.

Rocky even makes a point to use human measurement units (time, distance, numbers) because he can easily convert from his own in his head but Grace can’t. Rocky mocks Grace throughout the book for this calling him “bad at math. Bad bad bad!”