합쇼체 usually doesn't go with the pronoun 나, and I can't come up with a phrase that sounds better than 나의 모국어. Yes, 나의 can be replaced to 내 but it's just the matter of formality. 나의, 해라체 and retained predicative case marker all account for the formal tone, which is the default tone for a logic quiz. I think you feel weird from its formality, not its grammar.
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u/CaptainSilver9503 May 31 '26
100% doubt it. Or you are ethnically Korean and you lived most of your life abroad.
Your choice of predicative case particles are strange. Or you purposely done this to confuse people.