Sociolinguists have long been fascinated by the phenomenon of bilingualism and the complex language switching patterns that often accompany it. Many bilingual speakers are able to switch from language to language with ease, sometimes in mid-sentence. Attempts to define such patterns have not, however, met with much success. Research reports on the subject are cluttered with such obscure terms as 'diglossia', 'domain', 'code-switching' and 'ethnolinguistic vitality', but reduced to the level of a layman's understanding, the less than original conclusion would seem to be that choice of language is dictated primarily by the milieu in which the speaker finds himself.
A simple model of language choice might recognize the presence of only one factor in each category. It might be thought, for example, that someone with reasonable fluency in several languages would nevertheless choose to speak his mother tongue wherever possible, that being the language with which he is most familiar and comfortable. This natural tendency would be constrained by only one factor: linguistic congruity. A native speaker of Hungarian, for instance, might prefer to use that language wherever possible, but if he should himself in an environment where there are no other Hungarian speakers — in a foreign country, for example — he would be obliged to use his second or third language.
A simple model such as the above does, in fact, serve to explain a large number of language-choice events, but it is woefully inadequate in defining many others. Polyglots do not always prefer to use their native language, and the constraint of congruity does not apply to situations where the participants in a conversation share a knowledge of several languages.
A number of factual anecdotes may help to illustrate the complexity of the problem.
A group of people attending a party are holding a conversation in Polish. The Scottish hostess approaches the group to ask how everyone is enjoying the party, and the conversation switches to English. Presently the hostess leaves to talk with other guests, and the group continues to talk in English for several minutes. After a short pause in the conversation one of the members of the group reverts to Polish, and the rest of the group follows suit.
A foreigner visiting a Japanese department store approaches one of the shop assistants and asks for something in Japanese. The customer's Japanese is grammatically correct and well pronounced, and the assistant has no difficulty in understanding what the person has said. Nevertheless she chooses to reply in English. For several minutes the conversation continues in two languages, the customer speaking in Japanese, and the assistant persisting in a use of English.
On a flight from Honolulu to Tokyo, two young Oriental ladies are discussing, in elementary accented Japanese, the friends they will meet and the shopping they will do when they get to Japan. Their conversation is interrupted when a cabin attendant comes along and asks them, in English, whether they would care for some light refreshment. Both of them answer in fluent unaccented American English, and after the cabin attendant has gone to attend to their order, they switch back to Japanese.