The definition of fluency in a language is a very subjective matter. The following views come from my personal language learning experience as well as from my experience with students. I consider myself fluent in six languages and have been teaching these through London Language Studio for some time. I can understand and could speak several others but I somehow never mention these- I don't feel I am fluent enough....

 I am afraid the definition ‘fluent’ is almost an abstract and often misused term.

I believe that a good criteria to assess whether you are or not fluent in a language is to understand jokes. To be able to understand a joke requires not just advanced language knowledge but also considerable cultural awareness. Knowing a language means also knowing its people, their culture and mentality. A combination of sociolinguistic awareness, command of the language commonly used in day to day communication and a good understanding of universal grammar (including how this applies to each of the languages in particular) are the three important factors, I believe, which contribute to us feeling fluent and accepted by the society where each of the languages is spoken.