I've been asked this a number of times. I was asked it on CBC television a few weeks ago, and I tend to stumble through the usual platitudes about the fact that everything we do is within language. We organize our thoughts that way. We communicate our culture generation to generation that way. We are essentially shaped by the way we think about things, and the way we think about things is within our own first language. Some of us who are lucky and skilled enough to have more than one language—and I do not count myself fully in that group—are able to do that functionally in two or more languages, and that is a gift.
One of the things I thought, the last time I answered this question was to ask if you could rephrase the question without using language. Clearly, you couldn't. The fact that we need language to communicate is self-evident, and also the fact that, when we grow up learning a language to a level of fluency, that allows us to be eloquent in our language. There are gifted individuals who reach that level in more than one language, but it's not necessarily that usual. I expect the vast majority of us, whether we're bilingual or trilingual, still feel most comfortable in our first language, the one in which we communicate with our family and with our community.
Again, we could continue on and just talk about the knowledge that is inherent within each language, and how things do not necessarily translate, and that there's so much we don't know because we don't speak to the people who do know it in their language.