They didn't have to. But they also shouldn't expect the annexing government or populace to accommodate them.
Their country lost the war, lost the territory, and those that stayed and chose to take American citizenship should've learned English, the (de facto) language of the country they chose to join.
Their country lost the war, lost the territory, and those that stayed and chose to take American citizenship should've learned English, the (de facto) language of the country they chose to join.