First language vs Native language?

1. What are the precise definitions of "first language" and "native language"?
Are they interchangeable?

2. My cousin was born in Korea and the very first language she learned was obviously Korean; however, her whole family moved to the US when she was 7 and she grew up in New Jersey, USA. (She has been living in the US for nearly 20 years.) So she now speaks English much better than Korean.
The language she speaks best, most comfortably and confidently is English.
Is her first language still Korean? What about her native language? Her Korean became so rusty and she sounds like a brain-damaged person whenever she speaks Korean. Can we still call her a native speaker of Korean?

I dont think so. But: do you know if she also uses Korean (exclusively) with her family?

My cousin speaks Teochew and German natively, and while she uses Teochew only at home (with few relatives in Germany), I think she's competent equally in both AND in their respective language environment. Simply to say, I don't think she can use Chinese (written, I mean) properly when she's always faced with signs, documents, homework, ... classmates, the people on the streets... in German only.

I happen to be a Teochew, and know a few Teochew people here in Sydney, Australia. Impressively, there is this Cambodian-born, Australian-bred bloke who speaks the pure version of Teochew in contrast to me, influenced heavily by my Mandarin knowledge, as I grew up in China. I can see my language well preserved overseas in conversations with him, whose Teochew sounds exactly like that of my grandparents, who don't speak a word of Mandarin. However, I understand he speaks both Teochew and Khmer with his family, and English with others. And it's also in my understanding he's not capable of discussing on specific political or academic subjects in Teochew or Khmer, but English, for he has only familiarised himself with such set of meanings produced in English. I'm not aware of any commonly agreed definition of first/native language. But I can imagine for a multilingual person, whatever language he's native to, he may find a particular language to be more handy in a conversation in a particular area, than that of others. Say I speak several Chinese languages with a proficiency greater than I speak English. When it comes to academic disciplines, it's English in which I can express myself most effectively, because I go to Uni where English is the language of instruction, and it's the English voice in association with them in my head.

he may find a particular language to be more handy in a conversation in a particular area, than that of others.

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correction: he may find a particular language to be more handy in a conversation in a particular area, than others.

correction: he may find a particular language to be more handy in a conversation in a particular area, than the others.

>>And it's also in my understanding he's not capable of discussing on specific political or academic subjects in Teochew or Khmer, but English, for he has only familiarised himself with such set of meanings produced in English.

That's both good and bad news.

I'm also of the same group, but my grandparents 1) think Teochew is village talk, and thus vulgar, 2) don't want to teach my dad and me their first languages, so that they can keep it secret, and 3) they aren't used anyway in the SAR. While I can still claim weak ties with the capital of Cantonese, ... well, I'm de facto a member of a group that I don't always agree with - being a Hongkonger - and at times I have to make concessions between identities. So, you see, even a family that has moved about 100 km south already gave me multiple identities...

It's kind of a pity not to be able to have full functional command in multiple languages - I don't mean those you try to learn from scratch and books for pure cultural interest or practical needs - but those that belong to your identities (ethnic origin, country of origin...). And I even find people who can't maintain their native/first languages very well even in a place where it is commonly spoken/official. In this respect... the problem becomes very personal and depends on personal feelings.

I still feel guilty, but the guilt isn't really on me. Part of my identity, which is conscious in recent years, is simply not retrievable. Sometimes it feels as painful as having forgotten a native language in childhood, but actually it just never exists in my head, except a few phrases that are often so much repeated among the local folks, but often sound so vulgar that everybody remembers the utter poverty of those areas (and bandits these days)... and the utter useless-ness...

Nowadays, if you say "latino" in Spain more and more people will think in somebody from the other side of the pond. The classical language, of course, is "latín", the original people who spoke it "romanos" (whether provincial or not, in the case of the Iberian peninsula: "hispano-romanos") and the languages that have come from it "lenguas románicas" or, far less, "lenguas neo-latinas". If you say "los pueblos latinos", in plural, it will mean the peoples who speak Romance languages as their native tongue.
Anyway, the fact is "latino", to name a person or as an adjective ( "un latino"or "es latino"), has never been widely used in Spain and can, therefore, be considered techically as an anglicism coming from the US.
Ironically, those who have made the term more popular in Spain is the big South-American immigration we'have had since the 1980s. Furthermore, "americano", without another specification (hispano-americano, norte-americano...) tends to be identified, in Spain, with the USA.

Well consider this. I couldn't imagine a world without different ethnicities. I like variety. I love experiencing different cultures, different viewpoints, different phenotypes, different languages. I like how black people look so clearly different from white people, and all the different races for that matter. Each racial phenotype has its own beauty. If we didn't have different races, the earth would be as boring as all food tasting the same. It's thinking that one race is superior than another, or destined to exterminate the others that is alarming.