As is well known, bilinguals are extremely good at keeping to just one language when they are in a monolingual environment. If, in addition, they speak that language fluently and have no accent in it, they may "pass" as monolinguals (see here). How they deactivate (or even inhibit) their other language(s) is a question that is being actively researched both in psycholinguistics and neurolinguistics.
This said, one of the mysteries of bilingualism for someone like me who has worked on the topic for so many years is that, however well bilinguals "close out" a language, it sometimes comes through fleetingly in the form of dynamic interferences, that is episodic deviations from the language being spoken (or written) due to the influence of the other, deactivated, language(s). (Note that other types of interferences are static in that they reflect permanent traces of one language on the other such as a permanent accent; they often go by the name of "transfers").
Dynamic interferences–a bilingual's unwanted companions–can occur at all levels of language. For example, at the level of pronunciation, interferences may seep through if the person is tired or under stress. They materialize when you mispronounce certain sounds (e.g. the English "th" sound, the French "ou" sound), when you put equal stress on all syllables of a word which only requires one stressed syllable, or when you use an intonation pattern based on your other language.
At the level of words, we find those infamous false friends (the constant fear of translators and interpreters, among others) which correspond to near homophones or near homographs in two languages, but with different meanings. How many times have I not said, "librairie" in French (it means "bookstore"), basing myself on English "library", when I should have said "bibliothèque". (The same false friend exists between Spanish and English–a "librería" is not a library but a bookstore).
There are also those words that are unreliable friends such as Spanish "historia" which does mean "history" but which also means a story or a tale. Nancy Huston, the Canadian and French bilingual writer, reports that she ends up avoiding the use of false friends such as French "éventuellement" ("possibly" in English) and "eventually" to make sure that she doesn't mix them up.
Syntactic interferences happen, for example, when bilingual speakers use the word-order pattern of one language in the other, insert determiners where they are not needed, put the wrong gender marking on an article, or use prepositions inappropriately.
Idiomatic expressions are well known traps when they are translated word-for-word since all the words uttered are perfectly fine but the overall meaning is not. For example, "I'm telling myself stories" uttered by a French-English bilingual should be replaced with, "I'm kidding myself" (the person based herself on French, "Je me raconte des histoires").
It is at times like this, when the person being spoken to asks what is being said, or reacts in some way, or even offers the right expression, that bilinguals are taken aback. They were sure they were speaking correctly, and in the right language, but they suddenly realize that all was not totally clear.
Those bilinguals who write in both their languages have to be particularly careful with the spelling of near homographs. French-English bilinguals have to stop and think how many d's there are in "address" (only one in French), how many h's in "rhythm" (again only one in French), etc. The best present bilinguals received with the advent of word processors were spelling checkers!
If bilinguals are clearly dominant in a language, then it is their stronger language that influences their weaker language primarily. However, when bilinguals are fluent in both languages, interferences are often two way, with each language being able to influence the other from time to time. This leads some bilinguals to think that maybe they don't speak either language well when, in fact, these are very small bumps on a what is usually a smooth road.
We should keep in mind that once a bilingual has attained a stable level of fluency, interferences, if they occur, rarely compromise communication. As I wrote in an earlier post, they even render what is said more original, less stereotypical, and stylistically more interesting, as can be seen in the prose of many bilingual writers (see here). As bilinguals, we should maybe heed Barbara Kingsolver's words of wisdom when we think of interferences: “The friend who holds your hand and says the wrong thing is made of dearer stuff than the one who stays away.”
Photo of two women whispering from Shutterstock.
François Grosjean. (2012). An attempt to isolate, and then differentiate, transfer and interference. International Journal of Bilingualism, 16(1), 11-21.
François Grosjean. "Speaking and writing monolingually". Chapter 6 of Grosjean, François (2010). Bilingual: Life and Reality. Cambridge, Mass: Harvard University Press.
"Life as a bilingual" posts by content area.
François Grosjean's website.