I vaguely recall hearing two phrases about language, which were roughly:
1. Understand as the learned, but speak as the common. (This was apparently a translation from Latin.)
2. Unlike with morals, where our goal should be to act better than one's neighbors, with language our goal should be to speak just like our neighbors.
Unfortunately, I don't recall the precise wording, except to remember that it was considerably better than what I reproduce above. Can anyone remind me what the quotes were?
By the way, I realize that proverbs like this aren't always exactly right, and are in any event oversimplifications. Naturally, it's generally good to speak more accurately, more precisely, and more clearly than your neighbors -- the point of both sayings is simply that effective communication usually calls for a style that is familiar to listeners, and that doesn't distract or alienate listeners by departing too much from the norms of the listeners' place and class.