There is no fully agreed upon means make words ending s (x or z) into possessive - the below site gives example of Dumas' and Dumas's as valid.
My general response has been if it looks ok with just an ' leave it and move on. I can open my own shop with the left over apostrophes.
http://en.wikipedia.org/wiki/Apostrophe#Nouns_ending_with_si...
Still, I prefer my rendition. The problem here is the incompatibility between the rules of pronounciation -- French permits silent letters to end a word, English in general does not. In translating to English you must choose either to honour the French or the English rules of pluralisation and possession.
Because I don't write many English sentences with de la, du or d', I figure that sticking to the English rule is the best move.
edit:
> Jacqueses! (Jacuqi?)
Jacqueses would be closest (-ii is a Latin pluralisation, Jacques comes from Jacob which is Hebrew).
There's an impedance mismatch here: English stores plurality in the word, French stores it in the sentence.
Thank you. I changed it to the Americanized form of possessive for words ending in s (pronounced or not) I think this could go either way, but I screwed it up no matter which choice an author might take.