I think learning a language requires some abilities. First of all, you must be fluent in your own language. It might seem ridiculous but I know lots of people that can’t speak fluently in their mother tongue. What do I mean by fluency? Of course, even ten years old children can express their desires and feelings. But most of us can’t express him/herself in the slightly complicated topics such as politics, public speaking and even discussions…If you can’t write an essay in own language you can’t write in a foreign language. I think you have to have a good background in your own language. Otherwise, you are always a child for native speakers.
Steve once expressed his opinion that 'fluency in your own language" depends on how much we read in our own language. That is of course a factor, but it is not all. There are people who intrinsically eloquent in their mother tongue, and who couldn’t have read a single book.
When you know people well, say the family members, you have a pretty good idea how much they could have read in their life. And the correlation between how they speak and and what/how much they have read is far from being direct.
I recall a painter who always painted the walls of our apartment in Russia, and I as a boy observing his work for hours. His education has been 4 classes at school. He often boasted about earning good money without ever reading a book. I believe it was true, according to the essence of what he was talking about. But man, what a good talker he was! Much better that my father or me. Especially when the painter explained me about the women 
Iilya, I did not say that fluency depended on how much we read in our own language. Literacy does, and eloquence. The ability to discuss intelligibly on a variety of subjects is influenced by our literacy, and by the range of subjects we have listened to.
Native speakers are, by definition, fluent in their native language. Some are more eloquent or literate than others. All are fluent.
Thank you for clarifying your words Steve. Though Nasir might have implied the other definition of " being fluent in your own language", closer to " being eloquent, being skilled in expressing yourself in your own language".
@“If you can’t write an essay in own language you can’t write in a foreign language.”
I don’t think this is necessarily true. You can be able to do things in you foreign language/s that you are not able to do in your native language, it is simply a matter of what you’ve learnt in which language.
I guess this is especially true for people living abroad. Here are two examples:
If I would need to explain what I’m working on in Serbian, or even to describe a simple experiment, that definitely wouldn’t sound fluent. There would be a lot of ‘ahhhm’ and ‘uhm’ and searching for words and inserting English words, all that because I’ve never learned these scientific terms (or even names of the things around the lab) in Serbian, I’ve learnt them since I moved to NZ. Another example are the car parts. I’ve no idea how is e.g. cambelt called in Serbian, simply because I first time owned a car after I moved to NZ. I never had to think about these things in Serbian.
If a person can’t write an essay in his own language, it shows he hasn’t the ability to arrange his thought sequence. In other words, his subconsciousness is not full of with plenty words and proper expressions(like me). As we know, a tongue sounds a language from the deep side of a brain(do not understand literally).
Aineko, in your case, I think you may not consider Serbian as your native language. For me, being a native means a person’s do not have a single problem in his/her native language. I think you can consider as your native language which you’ve spoken during your dream.
In that case I have lots of native languages!
I agree with Aineko that unless we make an effort to find out how technical, environmental, scientific developments etc are integrated into our native language, we become ‘rusty’ in our mother tongue.
I have also found that when I struggle to find a German expression for an English word, the Germans are using their own variant of the English expression in any case, “gecuttet”, “geinputtet” ouch, ouch, ouch.
Nasir wrote:
“I think you can consider as your native the language which you’ve spoken during your dream”.
I think it’s true (unless you overtired with a new lang., then you might elicit it while sleeping). Spies and sposes, be cautious!
There may be a related definition - your native language is the one you think in when you have to think hard.
I knew a few scientists who were trained in their specific brunch of science in English, and who would have difficulty to discuss their work in their mother tongue. So they reasoned and thought in English, at least when you speak to him. But when they needed to think hard, to check themselves thoroughly and stay focused on their thought - I observed them to mumble the words of their mother tongue, interspersed with the English terminology.
There will often (always?) be areas where the vocabulary is limited. Each profession has its own terminology. You can’t expect to know every word in your native language. There might be situations where the target language is stronger than the native, in one aspect or another (listening, reading, writing and speaking), you might speak a target language better than you can write in your native, you might write in your target language better than you speak it (my German is like that).
In my opinion, being fluent in your native language does NOT mean that you (have to) know every word in the dictionary.
@“If a person can’t write an essay in his own language, it shows he hasn’t the ability to arrange his thought sequence.”
If a person cannot write an essay it just means that they’ve never been taught how to do it. There are examples of illiterate people who could be very good orators, showing that they can organize their thoughts very well.
“Aineko, in your case, I think you may not consider Serbian as your native language.”
what he hack is my native language then? 
" I think you can consider as your native language which you’ve spoken during your dream."
which are in my case Serbian and occasionally Russian and Spanish (interestingly, although I think in English 99.9% of the time (compared to Serbian, I’m not counting languages I’m learning), this language has never found a way into my dreams).
"There may be a related definition - your native language is the one you think in when you have to think hard. "
again, in my case, this is not true. In last few years all my hard thinking is in English, I never mumble in Serbian. But there are two situations where you can not mistake my native language: the language I swear in if you scare the sht out of me or if I’m so pissed of by something that I feel like trashing something or crying out of anger :). In these (rare) situations I will always swear in Serbian. In any other, milder situation, it will be English (like, even when I burn myself, it is always fuck!, not some relevant Serbian word ).
Nasir, you are mistaking fluency and native language for ‘knowing everything’. You cannot know everything that can possibly be expressed using one language. It is just to much knowledge for one human brain, given that “everything that can possibly be expressed using one language” is a result of centuries of work of many human brains. If I would ask you to precisely describe in your native language (or in any language) the process of appressorium formation on Magnaporthe grisea, right now, without checking any dictionary or wikipedia :), it’s highly likely that you will not be able to do it.
Definition of native language is simple - it is the first language that has wired language structures in your brain (pre-wired structures aside), the first language/s that you have learned from the state of not knowing any other language. As Steve already said, natives (without any mental challenges) are by definition fluent in their native language.
Fluency in foreign language is a completely different (and much more complicated) story…
I’ve known one of the greatest ways to cease a discussion, simply say: y-o-u a-r-e r-i-g-h-t :))))))))))))
By the way, I couldn’t explain my thoughts in the first post. May be I didn’t pick up the right word(“fluent”).
Writing an essay doesn’t mean “write about absolutely unfamiliar topics”. We are all human beings and we have similar basic need and desires. “The earth as my native land”, “What do you think what is a war?”, “How do think about an ideal human being, is it possible?”, “What do you think what is the purpose of life?” and so on. Most people can’t explain clearly. “hhhh”, "hmmmmm’', “I don’t know”, “uhhh”. These topics don’t require knowledge.
When I was at school I was learning dutch - well I can say that I was better in writing dutch than my native french - even if I did not understand what I wrote in dutch - the dutch I wrote was very correct - in the same time my french was full of mistakes. Most of my friends were in the same mood.
The need to be fluent in your own language to learn a new one - comes from learning from translation - which is less good than learning from listening and reading.
To be able to read in your native could be a difficulty to read in a new language because the risk is you read the letters the way you pronounce them in your native language. The same is true for the speech.
@"Most people can’t explain clearly. “hhhh”, "hmmmmm’', “I don’t know”, “uhhh”. These topics don’t require knowledge. "
but do require previous thinking, if you are after the immediate answer :). That is to say, these ‘uhhms’ could be caused by two different things: if someone doesn’t know the language or, in the other case, if someone has just never thought about it. The latter situation doesn’t imply anything about persons language ability.
In any case, I shall be paying a little bit more attention to how I use language, so some practical good has come out of this for me, in addition to the entertainment value of these discussions.