De Coussemaker founded the "Comité flamand de France" ("Flemish Committee of France") in 1853, which was tasked with putting a brake on the disappearance of the West Flemish dialect of the Dutch language, as spoken in French Flanders. With the priest Jules Auguste Lemire, he tried to maintain Dutch education in Catholic schools, especially in Belle, but due to the secularisation of education made compulsory by law, the Catholic hierarchy lost its influence. This brought a fatal coup to the survival of the Flemish dialect.