Before I went to Ireland, I considered myself as fairly fluent in English; now I do not. After I had been here for some time, I started wondering: When are you really fluent in a language? I can participate in conversations, even academic ones, and I have no problem with relying on my English. In spite of this, I still feel like a completely incompetent English speaker when someone uses idioms that I have never heard before.
A language consists of implicit cultural knowledge that non-native speakers do not have a chance to know about; even non-verbal communication within that language is different. Within different variations of the same language, difficulties of understanding can occur as the cultures are different from each other. For example, Americans on exchange in Ireland can sometimes find it hard to understand Irish, not just because of the accent, but because of the cultural differences; that is in spite of them both being derivations of English.
I do not think that language only consists of words. I think it consists of culture. To become fluent in a language, you have to be fluent in the culture. In another blog post, I argued that a language shapes your identity as you use the language to think. I still think that it is true, but I am now adding on to that. If the language is a product of the culture, so are you. You are a product of your native language and culture which your identity is bound to. I think it takes a lifetime to become fluent in another culture, just as it takes a lifetime to create an identity. With that being said, I still believe that knowing another language expands your identity; you become aware of another culture and another way of thinking.