I also have to disagree with the statement that the “[Norman] soldiers were unable to convert most of the Brits they encountered to the parlez-vous”. I’ve seen reasonable estimates of 30-40% of English vocabulary as being derived from French. Two personal experiences demonstrate the linguistic power of the Norman conquest. When I studied French in college, I came upon a reasonable heuristic when I was in desperate need of a word: choose a polysyllabic English word (non-compound and with phonotactics compatible with French, especially those ending in “-ility” or “-able”) and say it quickly with a French accent. It works more often than not. The meaning may be off a little, or a lot, but my instructors forgave confusing faux amis during conversation. On the other hand, when I took a course in Old English in grad school, I was shocked at how much more quickly other students seemed to be picking up the grammatical details of the language. I’m usually pretty good at picking up languages, and I was getting blown out of the water. I asked a friend and the class, and he told me that a few years of German made learning Old English a lot easier.

I know that the plural of “anecdote” is not “data”, but the stories do illustrate the point. The Normans roughed up our mother tongue pretty seriously, even if they didn’t completely replace it.