In one of the Indiana Jones movies we see a hieroglyphic portrayal of C3P-0 and R2-D2. Forget about how they ended up on Earth, they did it at least 3,000 years ago! THAT is a "long ago" in this context. So the most like scenario is the two droids had to eject from a ship on a life-pod just like they did at the beginning of Episode 4, but ended up falling through a transgalactic wormhole that drop them off in our solar system, where they crash landed on Earth in the vicinity of ancient Egypt. Assuming Professor Jones later found the two droids and was able to reactivate them, they must have learned enough English to tell him the Saga of the Whills. Jones' notes on the epic tale would have eventually been lost, but somehow ended up in the hands of a young film director, George Lucas, who used them as the basis of his film Star Wars...