Although a person who is "fluent" in a given idiom will invariably be able to express him or herself with speed and ease, these facilities are the manifestation, rather than the basis of fluency. The basis of fluency is simply the ability to "think in" a language...but what does this mean?Defining fluent might open up a can of worms here but by fluent I mean the ability to write and read in a way that isn't horrendously slow. Spoken Latin may also be added in here.
People on this forum seem to be able to write in Latin very well and be understood and reply to it. Yet when someone posts a translation request it often leads to pages of debate as to the best way of doing it, or even simply the correct way. How can those two situations be?
Humans, or rather human brains, actually undertake cognition without the involvement language, do not actually "think", in any language. Thoughts are any of three things: emotions, rationalizations, or memories; they arise as a consequence of the cognitive processes of: feeling, reasoning, or remembering. Language factors in later, after formation of the thought. For linguate humans, this is an automatic process, the mechanica of which is not fully understood. Even so, linguatization is a separate process from cognition.
The literary critic, philosopher, polyglot, and possible polymath George Steiner has indicated that every act of speech, even in ones native language, every act of linguatization, is a translational act, since it necessarily involves a translation of concept into the structures of a particular language. Though our thoughts do not form in language, once we have learned language we use it to particularize, differentiate, categorize and specify our thoughts, to relate them one to another, and to transmit them to other people.
The point is, that the automatic conversion of thought into (one of) our primary language(s) is translational-process-number-one (T1). The term "fluency" is used to describe that automatic T1 process. When we are leaning a new language other than our mother tongue, and our primary language is the language into which we automatically convert thoughts, we must then translate the concepts from our primary language into that language in translational-process-number-two (T2). Only when we are able to automatically undertake the T1 process in the learned language, can one be said to have achieved "fluency", to have become "fluent", in that language. This is what fluency means. It does not necessarily involve (though it is pertinent to) reading and writing, which involve yet a third transltional process (T3), that of translating phonemes or lemmas/words, into written phonetic or pictographic symbols, respectively.