What is fluent?
fluent is when you speak it unproblematically.
most westerners are fluent english speakers.
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''In the world I see - you are stalking elk through the damp canyon forests around the ruins of Rockefeller Center.''
Being "fluent" to me means being able to read and meaningfully analyze that language's literature, hold detailed conversations without anyone noticing that you are non-native (except for an accent perhaps), and write well.
Of course, by this standard many, many people are not fluent in their native language.
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WAR IS PEACE
FREEDOM IS SLAVERY
IGNORANCE IS STRENGTH
Normally "fluent" describes a command of a language much better than "basic knowledge", but less than "perfect" or even "close to the first language". It means that you understand texts, even complex ones, but you are not in the position to use the language in a way which would be regarded as written or spoken by a native speaker and lacks idiomatic expressions, but could be understood well and does not have any gross errors in grammar or spelling.
I always thought of it as 2 classes
Fluent = nearly indistinguishable between it and your first language. You can hold conversations, think in the language, and figure out new words easily by context. Others who consider that language their first language find you easy to communicate with and don't distinguish you from others who speak the language. Reading and writing and analyzing in the language are easily employed skills. Comfortable with idioms and customs of the language, and do not make mistakes based around those. (Knowing when to use certain sayings and certain phrases.)
Functional = able to carry on a conversation in that language and communicate all needs. Comfort level with the language is not as high as "fluent", though you can read, write, and analyze in that language. Conversation can be choppy on occasion, and sometimes you miss the right word. Simple grammar mistakes are made. When a new word is introduced, you might understand it or you might get confused. No great knowledge of idioms, phrases, and verbal customs in the language.
I consider myself "Functional" in French - in Quebec, I can convince most people that I am French Canadian, and they will happily speak in French with me for a while until they realize my speech isn't perfect and then they ask where I'm from. In France, people still switch to English almost immediately. I think I will never hit my definition of "fluent", because it involves too much non-verbal communication and non-literal understanding of phrases, etc. And that is extremely difficult for me. I'm content with functional, knowing that I can get by with the language I have, and while I might not be accepted for a native speaker, I will at least be respected for knowing the language, and can still get around in a country or region that speaks that language.
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~Izzy
In the realm of translators, interpreters, and linguists, "fluent" is not a popular word. Instead, there's a better (though far from perfect) scale used in the U.S. Known as the Interagency Language Roundtable Scale, it is used by the U.S. government as well as undergraduate and graduate programs around the U.S.
Full information here: http://www.govtilr.org/Skills/ILRscale1.htm
In brief, a score of 0 to 5 is given in the domains of speaking, reading, and writing, and a separate score is given for the ability to translate and interpret. In case you don't know the distinction, translators deal with the written word, and interpreters deal with spoken language.