When did the English kings start speaking English? In “The Lion in Winter” & various Robin Hoods, the king, queen & princes are running around using RP, but I don’t think that’s historical. When did English become the everyday tongue of the Plantagenets? The Stewarts? Did William III speak it? Did George III speak another language ‘at home’, as it were? I know French was the literal lingua franca of diplomats &/or aristocrats at various times & places, but when did English nobles switch to Eng.?

It is ahistorical but not as ahistorical as you might think. Henry II, the king portrayed in the Lion in Winter, supposedly understood English but didn’t speak it. 

The shift of the Plantaganets from Norman French speakers to English speakers was a gradual one: Edward I learned English as a child, Edward III was bilingual with French as his primary language but enacted the Statute of Pleading which changed the language of the courts from Law French to English, Richard II spoke English very well and his uncle John of Gaunt was a patron and drinking buddy of Geoffrey Chaucer, and Henry IV was a native speaker of English.