I thought we were talking about the AI's decision theory.

No, Shiminux and I were talking about (I think) terminal goals: that is, we were talking about whether or not we could come to understand what an AGI was after, assuming it wanted us to know. We started talking about a specific part of this problem, namely translating concepts novel to the AGI's outlook into our own language.

I suppose my intuition, like yours, is that the AGI decision theory would be a much more serious problem, and not one subject to my linguistic argument. Since I expect we also agree that it's the decision theory that's really the core of the safety issue, my claim about terminal goals is not meant to undercut the concern for AGI safety. I agree that we could be radically ignorant about how safe an AGI is, even given a fairly clear understanding of its terminal goals.

The implicit constraint of "translate" is that it's to an already existing specific human, and they have to still be human at the end of the process.

I'd actually like to remain indifferent to the question of how intelligent the end-user of the translation has to be. My concern was really just whether or not there are in principle any languages that are mutually untranslatable. I tried to argue that there may be, but they wouldn't be mutually recognizable as languages anyway, and that if they are so recognizable, then they are at least partly inter-translatable, and that any two languages that are partly inter-translatable are in fact wholly inter-translatable. But this is a point about the nature of languages, not degrees of intelligence.