I noticed something that hasn't been touched yet. You've given explanations on why you might be framing your thoughts in more serious contexts with English, but you have yet to demonstrate how casual and perceptive you are in other languages. Can you give examples of such and which languages?
Good question. Now I'm pretty sure I have to keep myself from falling into a cognitive bias of recollecting only such examples that side with either angle of the "formal behavior" theory I'm considering at that moment.
It would be fair to say that there is a handful of people that I know of with whom I have an easy time establishing a playful banter or cracking each other up. I have to say that the pool of such relations is restricted only to my natives as I haven't created equally strong relations with other nationalities.
As to the nature or style of those contacts they tend to be more intellectual, learned and scholarly by the virtue of our interests and backgrounds. In fact all of the people that I'd now consider my "long-term" friends, about whom I'm sure that they enjoy my jokes and we have a playful banter, have at least earned a master's (engineers) degree and are relatively highly cultured or established in that way. And I'm not saying it with any feeling of superiority, it is something that I hadn't considered until you've asked this question.
I'll try to break down one of the most recent quips that got us both laughing:
Essentially they went on a tangent about "Marian" and "Maria" names and the way the male form was feminine. Notably, phonetically "Ma ria" is equivalent to "ma ryja" which means they have a "face/snout" or "my snout" which is funny in and of itself. I said in polish "nothing wrong in derailing as long as you wake up in the morning with a blissful smile on twaryi, or there's a Twaryja lying next to you with the same blissful smile." "Twa ryi" here is used to mean "your snout" and Twaryja is capitalised to denote lying next to a girl/lover named Twaryja.
Anyway, I'm not sure if it's the kind of examples or explanations you're looking for. It's basic abstract wordplay with an innuendo, it forms a part of my jokes, but there are many other styles and forms of jokes I'm relying on.
What I meant by compartmentalization earlier was that perhaps you felt some subconscious desire to be formal when using English because it represents the academic/professional world to you, whereas you don't hold your native language and other learned languages up to the same high expectations.
Perhaps, I can make burp or fart jokes with one of my friends and it's no issue at all. Maybe I should find similar people in different languages to grow a sample and compare.
Also relevant, the idea of
high-low context cultures which is roughly thought to correlate with collectivist and individualist cultures. America and US English for example is low context(& high content) as in we put more emphasis on the words being said and require explicit verbal situations in our socializing. It means that misinterpreting signals and a lack of respect for unspoken rules are common motifs in the culture; the language doesn't reserve words for specific situations which ideally could impart great meaning, instead we string together the same commonplace words in different situations and hope our body language doesn't betray the idea we wish to communicate.
Thanks, I'll consider it, I don't have a meaningful answer at the moment.