Anglo-French

Anglo-French is the language of the Angevin Empire. It has developed since the Norman Conqust in 1066, so by 1964 there are regional accents of it throughout the Empire. Lord Darcy speaks it with 'a definite English accent', and Sean O'Lochlainn's Irish accent thickens when he gets excited or angry. Local dialects still exist, such that Lady Elaine, Marquise of Cherbourg could use sieur as a mildly pejorative term to her servant, Sir Gwiliam.

In the stories and novel, it is rendered as modern English. It would be interesting to speculate on what it might actually sound like.