Native Language or First Language

Do you find any difference in nuances between those two expressions? I feel that I hear "native language" more often than "first language."
Russian is my wife's first acquired language, and she is better at Russian than any other languages. However, she does not like it very much when someone refers to Russian as her "native" language because she is not Russian ethnically. Since her country became independent from USSR, her tribal language, which she acquired as a second language, is currently spoken in the country as the only official language. Aside from her emotional issues, does "native language" sound more ethnic-oriented than "first language" to the ear of native English speakers?

Not really. If I hear someone's "native" language is Spanish I wouldn't assume they are Mexican anymore than if I heard it was their "first" language. To me, it carries no more connotation of ethnicity than "first" language does.

Your native language, to be simple, NATIVE from Latin means BORN. That would easily mean that the language of your parents from which you were born and obviously that you First spoke and speak (maybe). Your First language then could be understood as that you have been speaking since you were born and speak most. But, with mixed couples and children nowadays, it is difficult, and the ancient colonies of Africa where people have a native language which is not internationally known (ethnic language). Which one would be their native language? French, English, Spanish....? or their own ethnic language?
In short, your first language is the language you speak most, best, and fluently than the others (if you are bilingual or multi-lingual) and your native language is your mother tongue (the one you were born with). Someone can be English and speak Japanese at NATIVE LEVEL, means like a native Japanese would speak it. But Japanese is not his or her native language nor first language. So, your wife needs not to be confused about it, let alone independence or else and assume the language of her parents as her native language and First language at the same time.
Hope it helps
P.

@Half-n-Half
Thank you! Great information for me.

@pic76
Thank you very much. I think I'm okay with the technical definitions of native and first languages. Rather, I wanted to know about what you reflectively feel when you hear those phrases, whether it's technically correct or not.

wonderful post from an expert and it will be a fantastic knowledge to us and thank you very much for sharing this valuable information with us.

English still dominates around the world though it is very fascinating to hear your language. I want to learn it!