Phyllis says people have often asked her whether her kids are thinking in English or French when they are speaking the latter, and I remember being asked similar questions when I was a kid.
I must admit I find it a puzzling question because I don’t think of thinking as being in a language most of the time. I’d say something like 4/5 of thought happens in swift, intuitive, pictorial ways, and it’s only put into words in a final output stage.
So when I’m speaking Spanish (just to take a language I’m fairly fluent in but where I have severe gaps in my vocabulary), most of my thinking is the same as the same as if I had been speaking Danish, but when I have to output it, I have to express myself using fewer words, and I will occasionally be unable to express exactly what I wanted to say. That doesn’t mean I have a word or a sentence ready in Danish, just that I’m thinking of a concept that doesn’t have an output mapping.
So I must say I disagree with Wittgenstein: Die Grenzen meiner Sprache sind nicht die Grenzen meiner Welt.