I've asked a close friend of mine something along those lines. His native language is Spanish but he's been speaking English was well since he was around 3, so I think he safely grew up bilingual.
Anyhow, he said his default voice in his head these days most of the time is English, unless he is around his family and is exclusively speaking Spanish out loud with them, in those cases he just naturally continues speaking Spanish in his head to himself as well without having to "think" about it.
But a lot of times when he's not kinda zoned out, or high/drunk/tired, his thoughts sometimes slip into being entirely in Spanish without him even realizing, and when he's reeeealllly drunk he won't even realize for a bit he's speaking Spanish out loud to me as well haha.
Anyhow, he said his default voice in his head these days most of the time is English, unless he is around his family and is exclusively speaking Spanish out loud with them, in those cases he just naturally continues speaking Spanish in his head to himself as well without having to "think" about it.
But a lot of times when he's not kinda zoned out, or high/drunk/tired, his thoughts sometimes slip into being entirely in Spanish without him even realizing, and when he's reeeealllly drunk he won't even realize for a bit he's speaking Spanish out loud to me as well haha.