I think holding onto Aquitaine is the best option. I once considered writing a TL on this but I know far too little about the 100 years war.
But if the English hold on somehow - we could see English starting as a "castle" language like in Ireland, moving to the commercial classes and eventually to everyone else. Maybe the Hugenots end up there after the French Wars of Religion or something. They'd be happy to learn English (as they did in London and New York).
But there is a difference between emigrating to an English-speaking country and learning the language to fit in, and undergoing language shift at home. Hanover did not become English-speaking despite being ruled by the British monarch for over a century.
Moreover, if the English monarchy holds on to Aquitaine, French might well remain the language of the royal court.
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