Until 1066, England's own language and literature were Anglo-Saxon (Old English), although Edward the Confessor had been raised in France and had brought some of that culture. In 1066, against the Norman Invasion, and at the decisive Battle of Hastings, Harold the Saxon King and the Saxon nobles did not successfully unite against William the Conqueror, Duke of Normandy (who had some vague claim to the throne). William's army of cadets, were largely recruited with promises of land, an attractive offer because of the Law of Primogeniture whereby only the oldest son inherits family lands. The Saxons were warriors but the French were soldiers, strategists, and administrators. The Normans defeated the Saxons and redid the real estate.
Normans brought their dialect of French so England immediately is trilingual:
French or Anglo-Norman might have taken stronger hold perhaps but for impetuous King John. In 1204, when trouble had arisen with one of his vassals in Normandy (for the King of England is Duke of Normandy), the vassal appealled to his overlord, Philip King of France. John would not come to court in France without an issue of "safe conduct"; he didn't appear at the inquest so Normandy was declared forfeited to France: hence his nickname, John "Lackland." Until then, nobles had double holdings and inheritances in both France and England. Younger brothers had come with William the Conqueror, but the older ones sat well in France (e.g., William's eldest son Robert inherited Normandy). Pre-John Plantagenets were more interested in France. Richard the Lionheart, despite Robin Hood legends which evolved from Saxon wishful thinking agaist their Norman overlords, said that the English are dogs, good for nothing but taxes.
In 1244 came the Decree of Two Masters. Louis King of France had vassal trouble and with all the nationalistic friction, declared that double allegiance is impossible. You either now stay in France and sacrifice your English property and English titles, or you go over. Henry III in England issued a similar decree. So families swapped and divided their holdings, ending the interlocking nobility. Former French-now-English nobility had to learn to cope. Thus we find a tremendous influx of French words between 1250 and 1300 as the language stabilized into Middle English: a filed down Old English with heavy French influence. The upper class now an English-speaking nobility again. Literature emerged in English (e.g., the romance Arthur and Merlin). By Chaucer's time, the royal court is using English.
One encounters in reading manuscripts some survivals of the old Saxon runic alphabet and characters we no longer use: