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Maybe some kind of inversion of Eloquent in My Native Tongue. But if someone is American-born, doesn't that make them literally native
to the USA?
Yes, this would definitely be better titled something like "Lacks Ancestral Language". And it's a common enough Real Life phenomenon, among recently-immigrated families worldwide, to be a "People Sit On Chairs". Unless it's about some bigot taking it for granted that a person must speak a foreign language just because of their ethnic background, in which case it's a Mistaken for Foreigner situation.
Is there a trope where a character can't speak their own native language? Such as an American-born Chinese being unable to speak/understand Mandarin or any other dialects.