I think I might be able to answer this one from my perspective. I was born in a Portuguese speaking country, so on paper my mother tongue should be Portuguese (which it sort of still is). But we moved when I was a kid and I lived in multiple countries, so I went to international schools for most of my life. English then became my mother tongue over time and it is dominant over Portuguese. I now work in an organisation where English is the main working language, but I live in France, so I acquired a third language, just not quite at native level. Here are some of the interesting things I’ve observed:

• I have slightly different “personalities” in different languages. This may be a reflection of exposure to different cultures and times of my life I learned these languages, but also very much a confidence thing. I am funnier and at ease making jokes in English than the other two languages.
• Some words I only learned in one language because of timing and circumstance. There are technical terms I know only in English because of my work. There are motorcycle parts I only know the name of in French because I bought my first bike here. I birdwatch, and for some birds’ names I default to English, while others I use their French name.
• Because of moving around I was exposed to a lot of different cultures, which is awesome, but that means I have cultural weak ties to my countries of origin (I’m also mixed race). If anything the one cultural constant in my life has been anglophone media (especially American) which had a mot of influence. I identify more with Anglo-Saxon culture but also feel vaguely European. I even sound generically American, which throws some people off when they learn I never lived in the US nor Canada.
• Knowing multiple languages fluently can obviously make it easier in some ways and make things more accessible. It also made me very adaptable. When I arrived in France I narely knew the language. Once I gained fluency, everything became much easier (well, as easy as this country can be).
• One disadvantage is that in some ways I am always the “other” (though not just because of language). Everywhere I go I feel like a foreigner, hence the username. I speak English to my kids, and that makes me stand out and people treat me as if I’m some sort of exotic being. It’s gotten better now they’re in a more international school.
• It’s harder to find people who “get it” because they lived through the same experiences.
• At work I sometimes have meetings with Portuguese speaking people but I’m uncomfortable speaking Portuguese in a work setting because I miss many of the terms. So I often default to English which confuses people because I’m from a lusophone country and I speak fluently.

There are some messier issues around identity that I won’t get into because those aren’t limited just to language, but the above are things that have stood out to me over the years.

Disclaimer: I am not a sociologist or a linguist, so everything listed below is purely based on my amateurish observations.

As most people around my age in Romania (Millennials), I studied English as my “primary” secondary language all throughout primary, grade, and high school. I also studied French, but that one didn’t stick to me quite as much (I kinda’ blame the teaching methods involved for this one, because I’ve slowly been reforming that skill by using French subtitles, but anyway).

What I’ve noticed with most of my generational peers is that we speak “Romglish” when interacting with each other (not universally applicable). It’s a very dynamic mix of both, for example: “Dammit, am uitat să mark it down în document, dar I’ll cover it până la următoarea ședință.”

From what I’ve seen, the tendency is to use the most poignant words or structures from both on a case-by-case basis - for example, when cursing, I’ve noticed that most people use English when they want something short and sharp, and Romanian when they want to flood their interlocutor with “well-wishes.” We also tend to favour Romanian when we really want to dig into someone, as Romanian curse words feel heavier than English ones. Romanian also has more variants for interjectional structures (I think) - when a “God damn it…” would be used, we use “futu-i morții mă-sii,” “băga-mi-aș” (with or without “pula”/“picioarele”), “Dumnezeii mă-sii,” “mama naibii,” “pula mea” (or “pana mea” as a lite variant), etc.

When talking tech, Romanian is mostly used to structure the ideas, while English is used to express the subject and related characteristics. It just “feels” more natural to use English for specialty subjects, as English techy words just sound more accurate and pragmatic.

In terms of dirty talk during sexy times, Romanian sounds weird/funny and kinda’ breaks the mood, so I think most bilingual people favour English (if they talk dirty at all) - this is rather speculative, as we don’t often talk about sex with eachother around here, but that seemed to be the consensus among the few people with whom I broached the subject.

When being intimate/vulnerable with eachother, we usually switch to Romanian, as it feels more personal and contains several structures which have more nuanced meanings than English - this may be a tired example, but “mi-e dor de tine” (which roughly translates to “I miss you”) is contextually more loaded with melodramatism, as it’s a mix of missing, yearning and craving, all wrapped together.

We still struggle with the accent - most English teachers back in the day favoured the English (I hope you Brits give me top marks for this ;) ;) ) accent over the American one, but few actually managed to reproduce it faithfully, so our accents are very specifically Romanian - they sound sort of Slavic, but we don’t modulate our vowels as much, and our consonants are significantly harder. It’s a blocky accent, for lack of a better word. Mine is a weird mix of English and American, with a bit of Romanian, Scottish and Irish thrown in there (mostly around consonants, and when capping off words - I use a lot of hard "r"s).

Speaking personally, my primary languages are Romanian and English, and they’re both native - started speaking English at about the same time as I did Romanian (~1 year old) thanks to having access to undubbed Cartoon Network, Discovery Channel, Animal Planet, etc. back in the 90s, and I was relatively conversant in both by the time I reached kindergarten age. We also had a lot of bilingual kindergartens back in the day, and so was the one which I attended.

As a result, I also think in “Romglish,” although English is somewhat more melodic to my ear, so I tend to favour it when expressing feelings, or when expressing complex ideas related to philosophy, tech, etc. When I talk to myself, however, I mostly use English, as I favour it over Romanian in general.

In my most vulnerable state, I use Romanian more frequently than English because it sounds a bit more personal, more intimate when used to express deep love or sadness, as mentioned above.

In general, however, I just use whatever’s shortest to pronounce (syllable count): “am o idee” instead of “I have an idea,” “just a sec” instead of “stai o secundă,” “got it” instead of “am înțeles,” etc.

Very late edit, because it may be relevant: most Boomers HATE us for this, with reasoning based on language purity (yeah…).

Comparing idioms is fun. “Great minds think alike” has an equivalent which is literally translated as “Idiots have similar thoughts”. Kinda reflective of the cultures too: self congratulating vs self deprecating.