Day 1 Story 1 of the Decameron is about the knavish Ser Ciappelletto, who after his death becomes reputed as the holy San Ciappelletto. In the Rigg translation which I'm reading, these names are left untranslated: Ser is not changed to Sir, and San is not changed to Saint. Why are these titles left in Italian in the translation? Do Ser and San carry some Italian contextual baggage that would be lost in Sir and Saint, or is it just for flavour?
Ser Ciappelletto cheats a holy friar by a false confession, and dies; and, having lived as a very bad man, is, on his death, reputed a saint, and called San Ciappelletto.
Ser Cepparello con una falsa confessione inganna un santo frate e muorsi; e, essendo stato un pessimo uomo in vita, è morto reputato per santo e chiamato san Ciappelletto.
(It's also interesting to note that his name was originally rendered differently in two parts of the same paragraph, and this is not preserved in the English translation. The variations of his name, however, are explained in the story, and I've also asked a related question about that.)