In the Aniplex English dub, when Caster mistakenly refers to Saber as Joan, he adds his French accent to the pronunciation of her name.
Now the wikia for Joan mentions 2 ways of spelling her name, Joan of Arc and Jeanne d'Arc. I don't know French all that well and for all I know, Caster could be saying either. So I am wondering, when Caster says Joan's name, is he using Joan or Jeanne?
Joan is just the English version of the French name Jeanne, both of which are female forms of the Biblical name John.
In the original Japanese, Caster refers to Saber as ジャンヌ jannu, which is the Japanese pronunciation of French "Jeanne". English "Joan" would be ジョーン jōn. I haven't seen the English dub, but I imagine that Caster is using the French version there, too.
Given that Caster is Bluebeard (i.e. Gilles de Rais, who was a French person), it makes sense that he would use the French version of the name rather than the English version.
