Anecdote: my wife immigrated to the U.S. from South Korea in her mid-20s with only a smattering of English and multiple college degrees in her home country. Si

Anecdote: my wife immigrated to the U.S. from South Korea in her mid-20s with only a smattering of English and multiple college degrees in her home country. Since then she's lived more or less in a 98% English only bubble, received more education here and has worked a steady job in various Engineering roles. Over the intervening 25+ years she's learned an uncountable number of new things and concepts over her life, but only knows the U.S. English language words for them (mostly technical, engineering, software, or workplace administrative topics). These topics also exist in Korea, or have arisen in Korea, and Koreans have given their own words and thoughts to these things. Sometimes they are loan words, sometimes not. Sometimes the loanwords are not obvious, not sourced from English, or filtered through an intermediary language like Japanese which put its own spin on it. So these days when she's talking with her family or friends, when they discuss something that showed up after she left Korea, they sometimes have to have a brief discussion to teach each other what to call it. Her other Korean friends who have been in the U.S. about as long as she has have similar struggles. She's also "lost" some of her higher-level native-sino-Korean words since she literally hasn't had to use any of it since college in Korea. Words like the name for an unusual philosophical movement during a historic period in Korean history for example. So yes, in some sense she's "losing" her native language as both the language co-evolves in two different geographies, and as she doesn't use some parts of it she's forgotten some complex vocabulary. But basic grammar structures seem to persist and day-to-day working vocabulary seems unphased. On the flip side, I don't speak Korean in any useful capacity, but know a few hundred words, and can read/write Hangul enough to get around. There are nouns and concepts I only know in Korean, or in the English translation of the native Korean terms, even if there are perfectly fine native English words for the thing -- mostly food words. Like "주꾸미", it's a kind of Octopus, but I have no idea what it's called in English. I also have adopted a kind of simplified English grammar when we're talking, or we're in Korea that seems to make me more understandable to the average Korean. I can see that in an alternate history, we had moved to Korea 25+ years ago I'd have most certainly lost an amount of fluency in English as my working language shifted to Korean.