Code-switching means when a person switches between languages in some kind of communicative event, like in a conversation, for example.
Many people are quite critical of code-switching.They seem to view it as something defective, as if people who code-switch have poor control or poor mastery of their languages. This kind of a view of code-switching most likely aligns with a monolingual mindset: there is one right way to speak a language, and mixing languages together shows lack of mastery in an idealized, "perfect" monolanguage.
But for people who speak more than one language, code-switching goes with the territory. There is a plenty of research on code-switching, and the consensus seems to be that code-switching is a perfectly natural and expected phenomenon. After all, we don't switch into a new brain when we switch into a new language. Our languages all exist within us as an individual; they are jumbled up in there together. One interesting point is that a person doesn't really even need to be bilingual to any great extent--in other words, they don't need to be equally or even close to equally fluent--to be able to code-switch. For example, in a classic study among New York Puerto Rican Spanish speakers, the researcher Shana Poplack noticed that even people who were not highly proficient in English still used the English phrase "you know" in their Spanish.
In Finland, where I live, lots of people frequently code-switch between Finnish and English. English is widespread as a foreign language here, to the point where it becomes quite natural for people to insert English into their Finnish if and when they feel like it.
Today I had an exchange at the pharmacy that went like this.
I scanned my ID card when I entered the pharmacy and approached the prescriptions counter. When I arrived there, the person working at the counter said, in Finnish: "You have a prescription for eye drops?"
This had never happened before--that the pharmacist knew what I needed before I even came to the counter.
I responded, in Finnish: Vau, mistä sä tiesit? Oliko ... magic? 'Wow, how did you know? Was it ... magic?'
The pharmacist laughed and did a funny thing with her hands, like she was casting a magic spell, and she said said, in Finnish: Joo, on vähan creepy. 'Yeah, it's a bit creepy.'
And then the conversation continued all in Finnish.
I found it interesting that we both code-switched between Finnish and English, but for very different reasons. When I spoke to the pharmacist, I actually had forgotten the Finnish word for magic, so I switched to English just for that one word. I felt confident that she would understand -- which she obviously did.
In her response, she followed along with what I had initiated, both in theme and in language choice: she used the English word creepy.
It is interesting to point out that, in her case, there was no language deficiency involved. Obviously, a Finnish person knows the Finnish word for creepy, but she was code-switching for social reasons. I see her use of the word creepy as a kind of politeness or accommodation toward me and the conversational tone I had established.
What is the moral of this little story? Well, lots of people are critical of code-switching because they think it marks a speaker as somehow being deficient or lazy. Such a view could accurately be used to describe my language use, as it was quite literally my lack of skills in Finnish that prompted me to code-switch. But, do you know what? Contrary to popular belief, this is actually the least common reason for code-switching, according to researcher on language contact (Yaron Matras, in his book Language Contact, for example, discusses this topic). Much more common in real-life language use is what the pharmacist did: she code-switched into English for social reasons, using her language resources and her social skills to suit the conversation and the interaction.
As an English speaker living in Finland, I feel quite grateful for this possibility to switch into my mother tongue and still be understood. And, if I am lucky, the people I talk to might think I am being socially clever rather than just showing my lack of proficiency in Finnish.