Outside of the Netherlands and Belgium, the dialect around the German town of Kleve (South Guelderish) both historically and genetically belongs to the Dutch language. In Northeastern France, the area around Calais was historically Dutch-speaking (West Flemish) of which an estimated 20,000 daily speakers. The cities of Dunkirk, Gravelines and Bourbourg only became predominantly French-speaking by the end of the 19th century. In the countryside, until World War I, many elementary schools continued to teach in Dutch, and the Catholic Church continued to preach and teach the catechism in Flemish in many parishes.

Answer this question, if possible (if impossible, reply "unanswerable"): It took until the end of what century for people in French cities like Gravelines to finally switch over to French from Dutch?
19th