"Antoine, how do you say "download a file" in French?
"Hem, I'm not quite sure"
"But I thought you were bilingual?"
How many bilinguals have found themselves in Antoine's situation, trying to translate a term or expression into another language, or attempting to describe something, or simply talking about a topic normally covered by their other language?
When I returned to Europe after twelve years in the United States and offered to teach an introductory course in statistics at my university, I suddenly found myself in trouble. I simply didn't know how to say simple things - albeit for someone who knows some statistics - such as "scattergram" or "hypothesis test" in French.
Antoine's predicament and my own can be explained quite simply: Bilinguals usually acquire and use their languages for different purposes, in different domains of life, with different people. Different aspects of life often require different languages. I have called this the complementarity principle.
If we were to take a bilingual's domains of language use, such as immediate family, distant relatives, work, sports, religion, school, shopping, friends, going out, hobbies, etc., and if we were to attach languages to these domains, we would see that some domains are covered by one language, some others by another language, and some by several languages. Rarely do bilinguals have all domains covered by all their languages.
I have met many bilinguals who have shared the experience of suddenly having to use a language that they don't normally use in a particular domain. It is often a frustrating experience. Bilinguals tend to hesitate and fumble in that language. They are often tempted to draw from their other language(s), a strategy that works when they are speaking with other bilinguals. When this is not possible, they may still borrow words but they then have to explain them. They may also quite simply shorten the conversation.
The complementarity principle accounts for many interesting bilingual phenomena. The first is language fluency. If a language is spoken in a reduced number of domains and with a limited number of people, then it will not be developed as much as a language used in more domains and with more people. This is true of certain language skills such as reading and writing as well as stylistic levels.
Well-learned behaviors are special cases of the principle - counting, doing arithmetic, praying, etc. - since one language usually has exclusive control of that behavior. How often have I had to think hard about my phone number in the "wrong" language!
Translation is another skill affected by the principle. Unless bilinguals have domains covered by two languages (as do professional translators), or have acquired their other languages via translation equivalents (I'm thinking here of traditional language learning methods), they may not have the resources to produce an adequate translation. Hence, even though bilinguals can usually translate simple things from one language to another, they often have difficulties with more specialized domains.
Children are also influenced by the principle. It explains, in part, why a language is more developed than another, and why children may switch over to the other language during a conversation, sometimes even in front of monolinguals.
Basically, the complementarity principle is one of the most pervasive aspects of individual bilingualism. Bilinguals live with it in good harmony until, on occasion, it interferes with their everyday language use.
Reference: "The functions of languages". Chapter 3 of Grosjean, François (2010). Bilingual: Life and Reality. Cambridge, Mass: Harvard University Press.
"Life as a bilingual" posts by content area: http://www.francoisgrosjean.ch/blog_en.html
François Grosjean's website: www.francoisgrosjean.ch