The Williams tried that with Norman French from 1066 onwards.
Oh, they succeeded well enough for their purposes. The barons used Norman French and, perforce, anyone who wanted to communicate with them had to know it as well. Many English (as opposed to Norman-English) thegns were bilingual until about the time of King John when English began to overtake French even at Court. But it lingered on for many years, at least until the time of Henry V.
And as your prize for bringing up "1066 and all that", here's a little poem which I learned as a child. Now, repeat after me:
"William the Conqueror, 1066
Said to his barons
I mean to affix
England to Normandy,
Go out and borrow
some bows and some arrows
we're starting tomorrow.
So William went conquering hither and thither
Till Angles and Saxons were all of a dither.
He conquered so quickly
you could not keep count of the counties he conquered.
Ten or a dozen - or maybe a score
And I haven't a doubt he'd have conquered some more.
But death put an end to his tactics, thank heaven,
Of William the Conqueror - 1087!"
FWIW
