Estoy aprendiendo espanol, y cuando no recuerdo una palabra o frase en espanol, I will just use English instead. No estoy seguro si esto is the same :P
I think I recognized some of the languages you used (English, French and a bit of a Chinese language, and Japanese), and I find it funny that I think I understand what you are saying:
You summoned a polyglot! I am learning/understand several languages, including my Spanish mother tongue. I like Galician and Japanese too!
Of course, false friends abound so I might be off the mark.
Speak, rarely, but when thinking, yeah, exactly what you describe. Also, sometimes there's a concept that's just described better in one specific language.
For speaking I have more of a “switch” going on. Besides, I'm always mindful of people around me not speaking my other languages. On the other hand… when talking to someone who shares more languages, I feel we can achieve some nice compression by mixing and matching.
My kid and I sometimes speak to in each other in a mix of three languages, and neither of us really notices when the other changes language mid-sentence. Doing the same with a friend I started hanging out with, with the same languages. I've noticed that I find it easier to speak about psychological things in English, and about gardening in Portuguese - that for each to pic I have my preferred language, but will throw in whatever word I cannot remember in another language. It's faster to communicate like that with my friends and family than religiously sticking to only one language.
I sometimes use Norwegian words in my notes & diary, but i mostly just speak and write in my native English. I'm only fluent in English so i don't use any particular language for any particular scenario.
My daughter, native German, fluent native-like English, and living in the Netherlands, usually communicates with a wild mix of English and German, and now starts throwing in occasional Dutch words.
Code-switching comes naturally. It's not even thought of as mixing languages since most people here know at least two languages, or even three. Tagalog and English are sometimes mixed, like:
Nakakapagod! Kailangan kong mag-overtime kasi gitpit masyado sa deadlines. (So tiring! I need to do overtime because deadlines are tight.)
I couldn't submit the paperwork on time kasi na-traffic ako. Ayun, naabutan na ako ng cut-off. (I couldn't submit the paperwork on time because I got stuck in traffic. I wasn't able to make it to the cut-off.)
But with the languages I am learning? Not really, unless I slip up, like when trying to show off: "¡Cuidade! Il ya un perro." I am not even learning Spanish.