What if anglo-norman replaced anglo-saxon as the language of England during the Middle Ages? What would it take for this to happen. What would the language look like today? What would it be called?
What I mean is that Anglo-Norman replaces Anglo-Saxon among the Peasantry.
1) A much larger segment of the population of England is killed or forced into exile following the Norman conquest, perhaps as the result of Norman reprisals after a successful southern revolt of Anglo-Saxons lead by Hereward the Wake.
2) Following this, the Norman Kings bring in a lot of French peasants to work the land. They also make active efforts to destroy English culture and discourage use of the English language. The remaining Anglo-Saxons are absorbed by the Francophone population and disappear as a separate group.
3) Some political event occurs in the 1100s to sever the political and cultural connections between England and Normandy for good. The Norman Kings and nobility of England gradually become more nationalistic and make active efforts to encourage differentiation between Anglais and French. Monks and other learned people in England begin introducing variations in the spelling of the language, which eventually translate into differences in the way it is spoken.
4) The Kingdom of England, for whatever reason (perhaps a side effect of killing or exiling such a large segment of its population following the Norman conquest, perhaps?) is less powerful in the ATL. As a result Wales, Scotland, and Ireland all remain independent, with their own languages. Cross-border population filtration cause some exchange of words between the languages which did not occur in OTL, which causes Anglais to diverge from French even more.
4) The process of divergence accelerates when printing comes on the scene, and the first dictionaries and grammars of Anglais are printed. Now the "language nationalists" in England can actively control the development of the language by controlling the content of the dictionaries, spelling, and grammar textbooks. By the 20th century, Anglais has diverged so far from mainstream French that it has become unintelligible to French-speakers, and it is a fully-fledged separate language.
Plus, you can't bring too many settlers across, the subsistence nature of most of the people in the day means that Britain won't be able to bear any more, and France will go to rack and ruin.