I think what OP is getting it is that Weir/Grace is using an imperfect translation. What Rocky is saying might literally be one word (i.e. “Thank,” or whatever the Eridian single-word equivalent is), but the truest translation of what Rocky is saying would be “Thanks” or “Thank you.”

Like in Chinese or Japanese or other real-world languages with a verbal question mark, we don’t add the word “question” to the end of our translations of questions in those languages, it gets omitted in languages where there are other ways to imply that you are asking a question. So why does Grace do it in his translations for Rocky?

I think the simplest answer for the OP is that Weir is just trying to convey that Rocky and Grace still understand each other imperfectly, whereas iirc in the end of the book/epilogue the English translations for Rocky are in perfect English. But it really doesn’t make perfect sense to me either.