If you lived on a border of a country that speaks different languages how is it chosen what language you speak? If you lived on the border do you just learn both languages?
Or is it more if you lived even like 500 meters of a border do you learn the language of the country your in? Do people choose it based on nearest popular city to where they’re at?
They speak what their parents and neighbors speak. This is constant even when borders shift.
The formal language they conform to is the nearest administrative region, usually in the country controlling the town.