Europe in the fourteenth century had a wide variety of languages and dialects, far more diverse than today. In real life, the characters in this book might have spoken any or all the following: several dialects of English including East Midlands, Kentish, Devon and Northern; North French, Norman French, Picard French and Mitan, a dialect of Walloon French; West Flemish and Brabantian Flemish; Middle Saxon, a form of Low German; several dialects of High German including Bavarian and Rhenish-Franconian; Czech; Occitan; the Tuscan dialect of Italian; Castilian Spanish; Cymraec Canawl (Middle Welsh); and for ecclesiastical figures and educated elites, Latin. Individual cities often also had their own dialects.
As a result, people tended to be quite polyglot, especially those who were better educated and well-travelled, and they could probably slip in and out of whatever language was required. Rather than attempt to replicate this rich linguistic complexity, we have rendered all their speech into modern English.
Place names have for the most part been given in the form most familiar to a modern English-speaker, hence Bruges, Ghent and Ypres rather than the more correct Brugge, Gent and Ieper. Calais was probably known to its inhabitants as Kales (West Flemish) or Calés (Picard); we have opted for the modern version.