That I have to ask 'how' merely proves that I'm a westerner. The manga approach would say 'It just *is*' and proceed to draw black French aristos duelling and sipping wine and writing tragedies in rhyming alexandrine couplets. The Japanese have no truck with the devil and hence never think to consider the details of anything-- the likelihood of a settei or its RL application, because it doesn't have one. 'There are people who can kill with words and they have totally human-appearing servants who are made of paper. Trufax. Now watch them have sex.' Western fen leave no approach unexamined, and god forbid that it be this-world historically unfeasible or scientifically inaccurate or, bref, that it couldn't happen that way.
So I mustn't think about how it might happen. (Some major plague wipes out most of Europe and half the Arab empire, explorers from Sudan or central Africa come up the Nile to the Mediterranean and cross to the fertile and depopulated lands of Greece and Italy; another wave navigates the western coast of the African continent and works its way across Gibraltar to Spain and then north to France. Successive waves of migrants follow them into the Iberian peninsula and France and onward to Germany and Eastern Europe unless-the-Mongols-are-there, settle, numerically overwhelm the surviving barbarians, and by the 18th century develop much the same society as the RL descendants of the Visigoths and Normans did. Mh. Is Christianity necessary for this at some point, or could it be assimilated into the mainstream African religions? To be French is it necessary to have been Roman at some point?) No, too complicated. It's not historical A/U, it's a Riverside other-world-entirely fait accompli sort of place.
In which the Marquis de Carabas is a *real* aristocrat, which was the point of the exercise in the first place.
(Mostly. The other was to have a society where the Chevalier de Saint-Georges was pretty much the norm, only perhaps considered a bit pale. BTW Saint-Georges fought a duel with the Chavalier d'Eon, which is something any mangaka would pounce on. Except that it was arranged and not an affair of honour.)