If an ESL person who is mainly English-speaking now is referring to his relationship with the English language, is English their adopted or adoptive main language? (Or are neither of the words correct in this context in the first place?)
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Adoptive refers to the thing doing the adopting, so you would be the language's adoptive user.
Adopted refers to the thing that is adopted, it is your adopted language.
I think it is fine to use adopted in this sense. It implies that you have taken the language on as if it were your own.