The Norman nobility (a better term perhaps than British in this period) spoke Norman French for several hundred years after the Conquest. In general, the kings of this period spent so much time in France and were so interconnected with the French royal line through marriage that it apparently took the upheaval of the Hundred Year’s War and the plague to cause of shift to English, something that presumably occurred in the reign of Richard II (1377-99).

It’s hard to pin down specific sources to show this, since it is rare for a text to say specifically that a king could or could not speak a certain language. Perhaps the closest is John of Trevisa’s translation of Ranulph Higden’s Polychronicon (a history of the world written in Latin). Trevisa added several comments in his translation that showed how things had changed in England between the time that he was writing in the 1380s from when Higden had written the original Latin text in the 1340s. One often-quoted paragraph describes how odd it is that children of gentlemen in England are taught French rather than English.

This apayrynge of þe burþe tunge is bycause of tweie þinges; oon is for children in scole a3enst þe vsage and manere of alle oþere naciouns beeþ compelled for to leue hire owne langage, and for to construe hir lessouns and here þynges in Frensche, and so þey haueþ seþ þe Normans come first in to Engelond. Also gentil men children beeþ i-tau3t to speke Frensche from þe tyme þet þey beeþ i-rokked in here cradel, and kunneþ speke and playe wiþ a childes broche; and vplondisshe men wil likne hym self to gentil men, and fondeþ wiþ greet besynesse for to speke Frensce, for to be i-tolde of.

Translation: This corrosion of the birth tongue is because of two things: one is because children in school, against the practice of all other nations, are compelled to abandon their own language and to construe their lessons in French, and so they have since the Normans first came into England. Also, the children of gentlemen are taught to speak French from the time that they are rocked in their cradles and can speak or play with a child’s toy. And rural men wish to liken themselves to gentlemen and so endeavor with great diligence to speak French, so that they are accounted more of.

John of Trevisa adds a comment to this:

Þis manere was moche i-vsed to fore þe firste moreyn and is siþþe sumdel i-chaunged; for Iohn Coruwaile, a maister of grammer, chaunged þe lore in gramer scole and construccioun of Frensche in to Englische; and Richard Pencriche lerned þat manere techynge of hym and oþere men of Pencrich; so þat now, þe 3ere of oure Lorde a þowsand þre hundred and foure score and fyue, and of þesecounde kyng Richard after þe conquest nyne, in alle þe gramere scoles of Engelond, children leueþ Frensche and construeþ and lerneþ an Englische .... Also gentil men haueþ now moche i-left for to teche here children Frensche.

Translation: This manner was often used before the first plague and has somewhat change since then. For John Cornwall, a grammar teacher, changed the instruction in grammar school from French into English and Richard Pencrich learned that method of teaching from him, and other men of Pencrich, so that now, in the year of our Lord 1385, and the ninth year of Richard II, in all the grammar schools of English children abandon French and construe and learn in English.... Also, gentlemen have greatly abandoned teaching their children French.

It was also in the reign of Richard II that poets like Chaucer started writing poems for the nobility in English. Before that, many texts written for the highest nobility of England (like Froissart’s Chronicles) were in French, although some poems (like Havelok the Dane), written for noblemen much less exalted were written in English in the previous century.

Your other question asking if the kings could have spoken English or communicated with the commoners is also a hard one to prove. There is a chronicle (unfortunately I forget which one, but you can probably find a reference in a History of the English Language textbook) that records an anecdote about Henry II (1154-89) speaking to a peasant in which he spoke in French, the translator translated it into English for the peasant, but Henry didn’t need the translator to understand what the peasant said back. I don’t know how much faith to put in that story, especially since Henry didn’t spend that much time in England anyway, but there it is.

Around the late 14th Century. Like virtually all instances of language change, it occurred gradually over several hundred years.

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