It will be well known to people in the Low Countries and historians of Dutch, but to others perhaps less well that for many centuries varieties of Dutch were spoken in what is now the far north of France. The family of the Anglo-Dutch poet, Jan Cruso, came from Hondschoote, and Michiel de Zwaan, an important poet and playwright in Dutch in the seventeenth century lived in Dunkerque, or Duinkerken. There are attempts to revive the language in schools and one still finds the occasional native speaker. A few years ago I was cycling in the area and was addressed by an older gentleman in Dutch. When I stopped and asked why he said 'Je lijkt op geen Fransman' - you don't look French! Apparently, he had spoken Dutch at home with his parents, but had to speak French at school and in public life. Perhaps the most obvious sign that varieties of Dutch or Flemish were spoken in the region is the many placenames that look Dutch rather than French. The lack of 'w' and 'k' in native French words indicates that Wormhout and Hazebrouck, to name but two, are of Dutch origin. Some places have Dutch and French names which are quite different - Ballieul in French is Belle in Dutch. Further south we find Arras, which has a Dutch equivalent of Atrecht. At first sight, the Dutch name for Lille, Rijsel, seems to have little in common with the French. However, Rijsel is a corruption of Ter IJsel, on or at the island, while Lille means 'the island'. It is heartening for lovers of Dutch to hear 'Rijsel' used in Eurostar announcements in the language.
For more on Flemish dialects including those of northern France, Veronique de Tier at the University of Ghent is an excellent person to contact.
See also M. Devos and R. Vandekerckhove: Taal in stad en land: West-Vlaams. Tielt, 2005.