The problem isn't quite so much "they can be translated" as... to translate them, you need to pause and first explain the concept. There is no existing conceptual token, phrase, meme, word or other sort of direct translation of the message for these Untranslatables, at least in the fiction, because their speaker did not explain them, they simply used the token.

The translation software (presumably) does not understand this stuff, and will not create new explanations where none was given by the speaker it is attempting to translate (again, presumably). For this, you would (presumably) require some form of powerful AI rather than complex algorithms - and the story preamble explicitly declares that AI never worked. I'm assuming this remained true for the other species.