why dows Cadfael an english mon use he French word jongleur?

Question

lingistic event in the English History occured in the century prior

Answer 1

Jongleurs were itinerant entertainers of the Middle Ages in France and Norman England. Since Cadfael is set in the Middle Ages, certain Norman-French words would be used, since they would have been "imported" into the English language.

Answer 2

Remember that England had been invaded and conquered by the Normans in the 11th century and so by Cadfael's time there would be many, many words of French that had passed into common usage by Englishmen.