My first language is French, but I'm living in an English-speaking area.
The language I swear in is generally the language of the closest social context at any given moment.
Eg: while I most commonly swear in French when I'm in a location where people speak French, because I hear the people speak this language, I may swear in English if I just read this forum post. The last language used is generally what comes spontaneously (like for swearing, dreaming, pain exclamations etc).