In fact the expression probably traces to the ages where the monks in the monasteries in the scriptoria were rewriting the Latin works and sometimes hit upon a Greek phrase (and almost nobody that that time in the Western Europe understood the classical greek) and they just wrote something as "
Graeca sunt ergo non leguntur"
(but I have no proof the English saying comes from that, but on the other hand... why not).
Edit: I'm not sure this happened everytime in those ages during the copying/rewriting... or maybe just in some monasteries. It is something I've heard from my teacher.