The language thing is interesting - it's a romance language - French (or Norman or whatever it was) became the language of the ruling class, so most words of an educated or governmental nature - education, government, assizes, felicitations, manor - anything else ending in -ion come from that, but we still retain the anglo saxon bits for the common everyday stuff - dog, log, gob, nob, muck, arm, leg.
So if you talked posh, Harold would struggle to comprehend, and vice versa with William.