This isn’t an argument as much as it is a suggestion, but I think we’re unlikely to find incommensurable conceptual schemes unless we come across a species that has incommensurable desires. Foucault’s Archaeology of Knowledge considers how concepts work without considering how they work in relationship to ends we want to accomplish and desires we want to fulfill.
I’m a pragmatist at heart: we want to do things with language and with concepts. And, as long as we find others who want to do similar things, we’ll probably be able to talk to one another.
Consider someone stranded on an island where no one speaks her language: it won’t take long for her to figure out some of the islanders’ words for “food”—cause if she doesn’t she’ll die.
If we ran across an alien species that was incorporeal, and therefore didn’t eat, poop, or have sex, then we might not be able to talk to them. But if we had desires that were similar to some of their desires (or that conflicted with their desires), I bet we’d find ways to talk pretty quickly.
I imagine Foucault might say that talk of the four humours is neither more nor less true than modern biology. But if we had a time machine, and went back to the ancient world where sick people were being treated for illnesses on the basis of that theory, I bet it wouldn’t take us long to convince them that our theory was better—simply because our theory would get better results.
Talk about incommensurability would seriously change, I think, if we embedded the discussion in talk of human desires, interests, etc.