If you are fortunate enough to speak more than one language, which one do you think in? This is a question that has never been satisfactorily answered, to my benefit.
As one of the tests I sometimes ask an unexpected question of someone who had command of more than one language and then note the first word of their response. But I’m still not satisfied.
Years ago a friend was the father of two young children. Many of their playmates spoke Spanish. In a short period of time they were speaking Spanish as well as anyone on the playground and speaking English at the dinner table.
Their father once told me they talked in their sleep part English, part Spanish.
I wish I knew which language they thought in. Perhaps both?