The first one you learned or became fluent in. For example, it's often English for USA people or Spanish for Spanish people, or Japanese for Japanese people
Often also called mother tongue or primary language.
Otherwise, it can be the one you are most comfortable with or default to.
It is a flavour of german talked in Zurich, Switzerland. I personally have some influence from cantons east of Zurich. There are no rules about how to write, we just write phonetically. Official stuff is written in German German.
Way off the mark then, embarrassing. Particularly since I'm from Southwest Germany, you'd think I'd recognise Schwyzerdütsch. I definitely need more exposure to dialects.
That is not just a regional dialect. And in my opinion dialects are more first language than the learning “real language” afterwards. That is clearly the second language…