When bilinguals are interacting with other bilinguals who share their languages, and with whom they feel comfortable, they may well intermingle their languages. (See an earlier post on speaking in a bilingual mode). They choose a base language and then bring in the other language when the need arises.
One common way of doing so is to code-switch, that is to shift completely to the other language for a word, a phrase or sentence, and then revert back to the base language. (Another way is to borrow a term from the other language; see here). The following code-switch example was reported by Aurelio M. Espinosa: "Vamos a ir al football game y despues al baile a tener the time of our lives." (The Spanish parts mean "let's go to the" and "and then to the dance to have"). Any bilingual reading this post can add one of two preferred examples from their own bilingual speech or that of others.
Even though it is widespread, code-switching has been criticized by some who feel that it is done out of pure laziness and that it is a grammarless mixture of two languages. Many pejorative names have been used to characterize this bilingual form of communication such as Tex-Mex and Franglais. Even the word "mixing" has now taken on negative overtones. One consequence of this is that some bilinguals never code-switch and may look down upon others who do, while others restrict it to a situation in which they will not be stigmatized for doing so.
Linguists have spent many years studying the intermingling of languages and one clear outcome has been that instead of being a haphazard and ungrammatical mixture of two languages, code-switching follows very strict linguistic constraints requiring good competence in more than one language. In addition, the cognitive mechanisms that lead to spontaneous code-switching are both complex and exceptionally fine-tuned.
Why do bilinguals code-switch? One reason is that some concepts are simply better expressed in the other language. Using a word or expression from that language allows you to be more precise than trying to find an equivalent in the base language. I compare this search for le mot juste to having cream with coffee instead of just having it black; it adds a little something to your drink, at least for those who don't drink their coffee black!
Since a bilingual's domains of life are rarely covered by just one language (see an earlier post on this), one will also code-switch to fill a linguistic need. You prepare a word or phrase in the other language and since your interlocutor knows that language, you insert it into your conversation. Another reason is to report what someone has said in that other language. It would be unnatural to translate it for someone who understands the language perfectly.
There are also interesting communicative and social reasons for code-switching such as personalizing a message, emphasizing a point, changing one's role (e.g. from friend to employer), including or excluding someone, emphasizing group identity, etc. An example of the latter was related by linguist Robert J. Di Pietro who noticed that Italian immigrants would tell a joke in English and give the punch line in Italian. Not only was it was better said in that language but it stressed the fact that they all belonged to the same community, with shared values and experiences.
In recent years, some Hispanic American contemporary authors have used the full richness of their two languages by resorting to code-switching in their prose. For example, Susana Chávez-Silverman, a professor at Pomona College in California, expresses her bilingual creativity by making extensive use of code-switches in a manner that is rarely found in the written mode.
Here is a short extract taken from her book, Killer Crónicas: Bilingual Memories: "Como northern Califas girl, of course, habiá visto mucho nature espectacular; the Pacific Ocean como yarda de enfrente, for starters, y los sequoia giant redwoods. Yes, especially los redwoods."
Listening to Susana Chávez-Silverman reading extracts from her book is an even more fascinating experience and shows how enthralling code-switching can really be. (Extracts).
(For a post on intermingling languages in children, see here).
Chávez-Silverman, Susana (2004). Killer Crónicas: Bilingual Memories. Madison, WI: University of Wisconsin Press.
Grosjean, François. "Code-switching and borrowing". Chapter 5 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