Well, if it exists, perhaps it could involve Buffy being magically or forcefully put into the role of a French maid in a strange alternate dimension or by a powerful villain. And the'slave' part might imply that she has to follow certain commands against her will while still trying to find a way to break free and use her slayer powers. But this is all just speculation as we don't know if such a fanfic really exists.
A possible plot for a 'Buffy French Maid Slave' fanfic could be set in a parallel universe where the rules of Buffy's world are distorted. Buffy could be transported to a place where she is made into a French maid and enslaved. Her slayer instincts would still be there, but she has to deal with the humiliation and restrictions of her new role. She might meet some allies among the other servants or captives who help her understand the nature of this new world. Along the way, she uncovers a plot by the rulers of this place to use her slayer powers for their own evil purposes, and she has to fight against all odds to regain her freedom and return to her normal world.
If there were a Buffy French Maid Slave fanfic, it might start with Buffy being captured by an evil entity that has a strange fascination with the French maid aesthetic. She could be forced to serve in a spooky mansion or castle. As she is a slayer, she would constantly be plotting her escape while having to endure the demeaning tasks of a maid. Maybe she discovers some dark secrets about her captor during her time as a maid - slave, which could be a key to her freedom and also to saving the day. However, this is just a wild guess considering the rather unique title.