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?
You learn what you have to and speak what is necessary at the time.
If you live in an area with a lot of minority language speakers, you should learn that minority language. If you are in a country that doesn’t speak your language, learn the local language as much as possible.
In the US for example: If you live in Southern Texas, you should know Spanish, but if you live in Fargo you probably don’t need to learn Spanish unless you work in an industry with a lot of native Spanish speakers and knowing Spanish will make things easier for everybody.
If you are fluent in a second language and you are talking with someone in your primary language, but it is their second language and they aren’t fluent, you should speak their language. Keep in mind that they may be wanting practice and may keep trying to speak their second language, take the hint and follow their lead. If both of you aren’t fluent in each other’s language, both of y’all struggle in both languages as needed to effectively communicate(This is honestly super fun).
There honestly is a big decision tree for multilingual areas like borders.