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.

I only really feel fluent in a language when people can’t immediately tell I am a foreigner.

In this particular case I believe the accent and pronunciation play a more important role than vocabulary or profound knowledge of grammar, at least in the first instance. I agree anyone can learn a language to a good level, but I don’t believe everyone can achieve an authentic accent or develop the ability to build on existing knowledge- which, in other terms, we might call ‘talent’.

In regards to how many languages one can learn, my view is simple; the more you learn the easier it gets. So someone who can already speak four or five languages fluently (in whichever of the above definitions of ‘fluency’) will certainly find it a lot easier to learn another four, and even easier, another four after that.

It just becomes a matter of time availability at that point.

However, I find that there is a certain limit to vocabulary capacity, where at certain point it becomes more difficult to learn and retain new words.

That is how my experience with German has been. I realized that between my knowledge of German and English, I could understand a good amount of Dutch, and now am learning it as well, which is another Germanic language. Sticking my head into the fog a bit more, I can understand some written Danish/Swedish/Norwegian and Frisian, though it is definitely more words and short phrases, as opposed to whole texts. However, if one I could put aside and learn the pronunciation differences, I think that I would have the heart of the language mastered, since these vocabulary sets share so much with what I already know. The biggest difficulty would be to learn to make the sounds to pronounce the words, but I’ve never heard of anyone who refused to learn English because they could not properly make the “th” sound, for example, so I think it seems doable.

I often think of my language abilities in terms of active or passive knowledge, in the areas of reading, writing, speaking and listening. At any given time these levels are in a constant state of flux. As a professional translator, my reading and listening skills across my professional working languages of French, German and Spanish always remains quite high (active). However my written and spoken skills in these languages can drop quickly into ‘passive’ mode if I’m not actively working to maintain them, because despite what people may think, I rarely use (or arguably even need) these skills in my work.

And then of course there’s the fact that I could describe technical issues relating to car systems in German, for example, or specific aspects of pharmaceutical research in French, more comfortably than I could discuss literature (an area which I wouldn’t take a special interest in in English) or even the weather (an area which I would!).

At the end of the day, the reasons why you are learning a particular language, and the contexts in which you see yourself actively using it, have a big impact on your degree of ‘fluency’ in that language.

Yes, modesty is always the best policy I believe! I totally agree that speaking 10 or more languages is something everyone could learn to do. As I mention on my site, I’ve originally got an Electronic Engineering background and never did well in languages at school. But when I decided to really try to, I could learn several languages. I do not consider myself naturally talented and always enthusiastically try to convince people of all backgrounds and claims of low linguistic abilities, to jump in the deep end of language immersion and give it a try! If I can do it, anyone can

I didn’t mean to say that capacity and natural abilities are the defining factor, just that some people really do have better capacity than others and require less of an investment for the same results. I try not to say this much since it doesn’t apply to me and it is a great excuse for people to lazily decide they aren’t smart enough to learn other languages. It’s a weak argument, as you say, for many learning aspects of life, to simply give up saying that you can’t. I prefer to give everything a try; there are no limits to human learning capacities, especially for “ordinary” folk like me!