William claimed the existing title of King held by Harold. French became less popular in large part due to the 100 years war. Henry IV was the first to take his oath in English. Henry V was a native English speaker.
The last thing William wanted was to invent a new title and leave the title "King of England" open to be claimed by one of his many enemies and rivals. The whole point of the Norman conquest was to claim the title "King of England" for William.
