In my view, this is a French work. The headdress is a Fontange, the dress is a Mantua, and this combination was for a short period (1690-1710 or thereabouts) extremely popular among the very wealthy French aristocracy, while being virtually absent in other countries.
I have been unable to identify the sitters or the artist, but the work is nevertheless very interesting as a piece of social history, depicting a peculiar but strangely impressive bit of fashion. There are many engravings showing this mode, but relatively few full-length paintings, and then they usually focus solely on the woman.
The painter isn't one of the top portraitists of the time but a good lesser name, someone like Simon Dequoy (1655-1727) who painted this very similar portrait of Anne de Souvré (in the Chateau of Versailles).
UPDATE: it was sold at Sotheby's in 2020 as "French school, ca. 1700" (yay, I was right!) for £11,875, nice! https://www.sothebys.com/en/buy/auction/2020/the-rafael-valls-sale-online/french-school-circa-1700-a-portrait-of-an-elegant







