It is my experience that fluent non-native speakers sometimes know a language better than most native speakers of that language.
Two examples: my wife's native language is Konkani, the language of Goa. Growing up in Kolkata, she spoke Konkani at home with her mother, Bengali and Hindi outside the house, and heavily accented Indian English at school. Now that she has spent thirty-five years in Canada I think she speaks fluent and unaccented AmE as well as a native speaker.
We have a friend in Mexico city who is a corporate lawyer. His native language is Spanish, of course, but as a corporate lawyer he relies on his excellent knowledge of English to earn his living. Occasionally he will make a small error. For example, one day I was quizzing him about the Mexican drink pulque and he said it was an acquired flavour when he meant an acquired taste. But on the whole I think his English is as good as mine, and I am a native speaker.
It is interesting to consider the people who do simultaneous translation at the United Nations and similar places. There are special colleges that train such people, and one of the friends of my youth attended one of them. His languages were English and French, and he was beautifully fluent in both, but English was his mother tongue. He told me it is an absolute rule of simultaneous translation that you translate only into your mother tongue. So I guess some experts disagree with me about non-native speakers gaining absolute fluency.
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