How about Fermont in Quebec, then? http://www.toponymie.gouv.qc.ca/ct/ToposWeb/fiche.aspx?no_se...
It is predominantly French speaking, is 1° further north than Dunkirk, is incorporated as Ville de Fermont, has a population of about 2400.
It also has a Wikipedia page that claims "Fermont is arguably the world's northernmost Francophone settlement of any considerable size": https://en.wikipedia.org/wiki/Fermont.