How do decide what language you speak living on a countries border.

You realize that speaking a language involves talking to another person, right?? So you speak what the interlocutor speaks in any given situation... and if both parties speak both equally well, then they generally speak a mix of both, based on whim or topic.

Such people are sometimes called Grenzemensch (border person). They grow up speaking multiple languages and don't even realize til they're older that the languages are different. They just think you have to talk to Uncle Fritz one way and Grandma Mireille a different way.

Language isn't defined by borders. What people speak in an area is what they grew up speaking and have learned to speak.

I mean... Babies and small children don't "choose" what language to learn, they just pick up whatever's spoken regularly around them. So whatever their families and community speak, same as everyone else?

I speak a little Spanish, but no Portuguese, and my time in Brazil right near the border with two Spanish speaking countries was a challenge, because most folks there didn't speak Spanish despite the location.

Some folks do of course, but most people I encountered didn't have any Spanish

I have zero idea how representative that is of other borders

Speaking both seems like a great choice.

Most people who live at the border tend to speak both, specially if people cross the border everyday to work

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.

Or is it more if you lived even like 500 meters of a border do you learn the language of the country your in?

That tends to be how it's done. States tend to be rather protective of their official language, and it's generally impractical to send your children to school in a different country. Being somewhat proficient in the other country's language is quite common, but to truly be bilingual you pretty much need to be some kind of ethnic or religious minority.

Also depends a lot on the relationship between the countries and languages; some borders are easier to cross, and some languages easier to learn.

On the colombian brazilian border they speak a mixture of portuguese and spanish called portuñol.

Depends on how much interaction there is qith the people across the border, but it's common to know (not learn, since you naturally acquire it) both.

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.

Depends heavily on which border, I think. I live in eastern Austria and don't know any Czech, Slovak or Hungarian despite those countries being within an hour's drive of where I live. For the most part I expect people in Austria to speak German, I don't expect people even in border villages of Czechia, Slovakia or Hungary to speak German; if they do, that is a nice surprise.

Some years ago, in a border village in one of these countries, a child spoke to me in the local language; I tried speaking in German and English, but the child didn't understand either, I could barely say "I don't speak (your language)" in that language. Eventually I figured out he was trying to ask me how old I was, and I could show that on my fingers...

I guess their mother decides that

If you take the time to learn the languages spoken in your area, you multiply the opportunities that will come your way, socially, professionally, romantically, etc.

I live in Ottawa, ON, Canada which has Gatineau, Québec on the other side of the river. A lot of people here are bilingual and will just switch to whatever language works better for the person they're talking to. The default language on the Québec side is French and the default on the Ontario side is English

Whatever the majority of the people in the area are speaking. Another factor is which side of the border the community interacts with more. If say the city is technically closer to an urban area on the bordering country than it’s neighbor within its own country, then they likely interact more with the former so will speak that language more.

Depends on how much interaction there is qith the people across the border, but it's common to know (not learn, since you naturally acquire it) both.

If you are Swiss, you just speak all the languages