I speak 3 languages regularly (Swiss German, Spanish, English), 2 of which I learned from my parents at the same time when growing up (the first 2 of the list; my mom would speak in one, my dad in the other). In my case, I swear in the languages where I used to swear the most in a given context.
For example, when I'm driving and I'm surprised abour something or a situation requires insulting I use English (I drove the most when I lived in the US). But when I hit my hand against something, slip and fall, or want to insult someone, I use Spanish (not sure why, maybe because I used Spanish the most in a casual/street context). When I'm in an unfriendly discussion I tend to use Swiss German words (maybe because my mom and I used to discuss a lot :S).