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.
I was under a vague and uninformed impression that many of them spoke Latin to each other? I'm happy to be corrected.
My main point was that an order from the top is not enough to ensure a particular language survives (cf the Academie Francaise - or on second thoughts maybe we don't want to go there again. Perhaps we could try the Vatican, whose wise citizens I understand to have been credited with the coinage 'bombus atomicus'!)
BTW, I love the poem!
