Voldemort also saves Harry from the Lethe Touch in Significant Digits, though how he knew of the spell was confusing to me (I suppose he may have learned from Salazar Slytherin's basilisk, or some other random place, and the Interdict of Merlin prevented him from actually casting it)

I think that they possibly evolved to have Untranslatable 2 whenever they disagreed which would perhaps resolve this.

Being unable to do so with another species, they expected a species which cannot have untranslatable 2 to be unable to come to a rational agreement.