technically... she is a princess. Princess William of Wales. Fergie was technically Princess Andrew of York. She is not Princess Catherine because she wasn't born a princess. *sigh... nerd award of the day.
Duchess does not mean princess. It's a completely different title. Maybe she'll become princess someday.
Also, nothing to do with Dutch (= language and people of the Netherlands).
Which means princess because it's the Dutch meaning prince William and dutchess princess Catherine