French was the dominant language amongst the ruling class.
Then in 1066 A.D., William the Conqueror (the Duke of Normandy) conquered England and took the throne. Under his reign, there was increased exposure and influences of French culture and language. French was the dominant language amongst the ruling class. Old French was used for the majority of the court and administration texts. As for a written language, Latin was mostly used, especially when it came to literature from the Catholic Church.