After 1066, when William, 7th Duke of Normandy invaded England and won the Battle of Hastings, the ruling class in England spoke French.
Not all of King William's barons were from France, but his most trusted friends were from France, and not just Normandy. His strongest and most loyal supporters were those whom he sent to guard his borders with Wales and Scotland. So on the borders, people might be speaking Cymraeg, or Gaelic or French or Anglo-Saxon, depending on who was doing the talking. (I wonder how they made themselves understood to each other.)
Maybe I would have worked harder in my French classes in high school if I knew I had French ancestors. But I never suspected such a thing. I "knew" my ancestors were all Finnish, except the ones who were Scots, Irish, English or German.
It took me most of my life to discover my "English" ancestors were actually from Wales, and now I find that the rest of them weren't really English either - they were French!