Actually, what he said made sense. See, Prince Philip is a highly educated man (yeah, I know, we wouldn't be able to tell by his famous quotes) and does speak perfect french. Not any french, french from France. Here is Quebec, we speak "joual" or french-canadian, which French people (from France) can barely understand. And yes, he was right, we do slur all our words!
I'm French Canadian. He's right - many of us speak atrociously. I suppose it could be considered an accent of sorts, much as Parisian French, or a Cockney accent can nearly incomprehensible to me. Still, The Prince has a point.
When Londonians speak, I don't understand a word of what they say; its called having an accent. French Canadian have an accent; who doesn't? International competitions prove it regularly, the quality of french in Quebec leaves nothing to be desired, as opposed to their sense of pride...
As for the prince, he probably didn't mean to, but he was rude, impolite and foolish.