I was raised bilingual French-English, and when I learned Spanish I was thinking in Spanish almost instantaneously.
For Japanese, I’ve found that I can take reading/listening at face value without translating. I can read a sentence and just grasp its meaning conceptually (as long as it uses words and concepts I know, of course). I’m not thinking “oh yeah, this part means X and that other part means Y”
But I’m not there yet when expressing myself. I just don’t have these neural pathways developed yet, so I have to mentally search my grammar and vocabulary database when constructing sentences. So it’s not like I’m making up a whole sentence in English in my head then translating, but I’m probably conceptualizing each individual element in English first to fish out its Japanese equivalent in my brain. I then construct my Japanese sentence from those Japanese elements I fished out. But that’s just something I’ll get faster and more natural at the more I do it. The majority of N5-ish grammar comes to me completely naturally, for instance.
I grew up monolingual, but learned Japanese in Japan. The way I learned was all in Japanese from Japanese teachers who didn’t speak any English. None of the text books had any English in them either, so sounds and words I learned their meaning directly and not by learning their English equivalent. Typically we’d get pictures with the word underneath it and verb forms would be explained with body motions, miming and more pictures. Four hours a day five days a week using this method and limiting how often I hung out with foreign friends never gave me an opportunity to have to translate anything in my head.
Eventually, as I think is true of most people who live in Japan and learn Japanese, talking to people who knew both languages I would flip back and forth between English and Japanese multiple times in a single sentence using whatever words came to mind first when speaking. Then in other situations I’d use whatever language the speaker would prefer.
The only thing with this method is the higher level you are you have to translate some words. Like names of diseases maybe some legal terms for getting a house and other such. But I would say up through N3-ish I did not have to look up many, if any, words directly.
So I had a really good situation for learning Japanese, I’d say.
I have been thinking in foreign languages a whole lot, my whole life (a slight exaggeration). Like onw ofvthe previous posters I also started reading English novels around 11. I also learned some Spanish, so that language also sometimes pops up, but I am not that strong anymore due to a lack of use.
As for Japanese, I am constantly building phrases in my mind. And then I realize I don’t know what an ironing board is in Japanese and get stuck, shrug and move on. So partly it is a mental exercise, partly just running commentary.
Also I speak to the baby I look after most days in a mix of Dutch, Japanese, English and Spanish. He likes it when I speak Japanese mostly, because that is where I probably put on the cutest voice😁 Talking to a three month old is pretty much thinking out loud.
The other day I was riding my bike in a park, and a giant dog came by. 大きい犬だ！I exclaimed😆