I toggle 2 languages all the time in my day to day life, because my beloved only speaks English.
If I'm having a conversation in English, I swear in English.
If I'm having a conversation in my native language, I swear in that.
Whichever language I last used (be it in conversation, text, or song) tends to be the language that stays "on". So even when I'm alone in my thoughts, the language that is "on" is the one I think and swear in.
Oh man, I hope what I wrote makes sense...