Come to think of it, I do know how to do it...

Suppose one makes two lists of all the propositions in a given language, say Japanese. One list would contain all the true propositions; the other, all the false ones. The lists would be infinitely long, of course, allowing for arbitrarily complex expressions. It would clearly be possible to do it, since one could sort the propositions alphabetically.

Do the same thing with another language, say Hopi.

Now translate all the true Japanese propositions into true Hopi propositions by matching up the two lists. And translate all the false Japanese propositions into false Hopi propositions by matching up the two lists. Such a translation would preserve the truth value of each statement -- even though meanings would become totally lost.

It would clearly work for any two languages that allowed for infinitely many propositions. It would also work for any two languages which allowed the same (finite) number of propositions.

I'm not sure this is what Lakoff had in mind, but it seems to match what he wrote...
Two conceptual systems are commensurable if each language can be translated into the other, sentence by sentence, preserving truth conditions.
~ Page 322.