Cornish is a P-Celtic tongue similar to Welsh but closest to Breton. Even as late as the 17th century the link between Cornwall and Brittany was strong.
Joan of the Tor is bilingual. The first two words by her in the novel are ‘Comely Vean’ or ‘little comely’. The adjective is the late Cornish form of ‘byghan’. Joan’s father utters no intelligible word to Marvel because in all probability he is a Cornish monoglot. The generation following Joan would have had Cornish as a second language if they had it at all.
Most of the soldiers fighting for the Royalists at Braddock and Stratton came from western Cornwall and would have had Cornish as their first tongue. Their regimental commanders would have had English as their first language but enough Cornish to facilitate communication. Hopton and Berkeley came from outside of Cornwall although no doubt they picked up something. Even the King had a letter to his Cornish subjects written in the native tongue.
There is a curious reference to Cornish in Chapter XVI. What put fire into the final charge of Sir Bevil Grenville’s regiment was a Cornish song started by Anthony Payne, the Cornish giant, and taken up by the men. As Payne came from Kilkhampton it is unlikely that he knew Cornish but I suspect that Q had some basis for this story.
The Killigrews would have spoken some form of Cornu-Breton, as would Billy Pottery who regularly traded with Brittany. Fowey traders continued to send their sons to Brittany into the 17th century so as to have a good command of the tongue.
The Puritans despised the Cornish language because they saw it as looking back to Medieval Catholicism and the Anglican Church refused the language in its services. Killigrew of Gleys would have been an English speaker although inevitably with some knowledge of Cornish. The families of Robartes, Carew and Buller would have espoused English. This possibly accounts for their failure to raise Cornwall for Parliament. In Devon, where spoken Celtic had long since ceased, this problem did not arise.