Do they? I don't think there are good records of 11th-century Norman French. But Geoffrey Chaucer wrote in 14th-century English and he is not intelligible to a modern English speaker.

(More accurately, he is intelligible in large part! But if you try reading one page of his work, you'll notice some serious problems.

http://www.sd-editions.com/CantApp/GP/ will even read it to you in reconstructed pronunciation.)