vulgarisateur (n.)

1940, a French word brought into English by John Buchan (Baron Tweedsmuir) and picked up by philosopher C.E.M. Joad because they found no adequate word in English for one "who spreads with clarity, vividness, force and accuracy, the knowledge obtained by and the wisdom derived from others" [Joad, 1948], vulgarize already being in use in the pejorative sense; see vulgar.

It has been pre-eminently the age of the vulgarisateur in the best sense of that word. I think the tendency wholly admirable. Lord Rutherford used to say that no conclusion which he ever reached was of any use to him until he could put it into plain English, into language understood by the ordinary man. Attempts to present the history of the world as an interrelated intelligible process, or to give a bird's-eye view of the long march of the sciences, may be faulty in detail, with many arbitrary judgments, but they do furnish principles of interpretation which enable the reader to find at any rate one way in the world of thought—perhaps a little later to make his own way. In this task the vulgarisateur may be preparing the soil for a rich future harvest, just as the work of the Sophists cleared the ground for Plato. [John Buchan, "Memory Hold-the-Door," 1940]