On the trip to the Engineer's world, David was learning to speak their language. But where did the lessons come from? On the star maps that were left, they showed a few symbols, but a) there don't seem to be enough for an entire language, and b) how would they know the pronunciation of the symbols?
David wasn't learning to speak the Engineer's language, he was learning ancient languages in order to be able to decode the Engineer's language. This is based on the theory that human languages are derived from the Engineer's language.