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.