In my family, we jabber away in two languages, using both English and Spanish interchangeably. Fluent in both, it is easy for us to substitute words, phrases, and clauses in one or the other language.
We don’t notice what we do because to us the two are easy on our tongue. In our brain they are one.
Someone will ask a question in English and get an answer in Spanish. Someone will make a joke in Spanish and all the rejoinders will be a mix of both languages. Gossip about a relative is discussed at great length in both English and Spanish and no one notices, except for those who get caught in the middle of our barrage and speak only one of the two. Even then, they are able to keep up because it’s like listening to one side of a phone conversation.
This phenomenon among fluent speakers of two or more languages is called code switching. It is a natural linguistic outcome when they share a similar syntax or phonology, like English and Spanish, but it is not unique to just these two languages. To code switch, the correct words, phrases, clauses, or sentences in compound structures must be interchanged according to the correct grammatical rules that exist between the two.
Code switching is not Spanglish – where a third “language” is created by defiling the two. When someone adds the letter “o” to the ending of a word in English or when a Spanish noun or verb ending is added to an English word to make it sound like Spanish, that is Spanglish, words like parquear for to park (a car), carro for car, troca for truck, marketa for market, lonche for lunch, tochar for to touch, etc.
Spanglish words are not correct in either language and someone who uses these is not code switching, but attempting (poorly) to speak in Spanish.
I love to eavesdrop on people who are code switching. It is so easy and so natural for them. They are unaware that their brain is dancing between English and Spanish and that they are creating one language out of two.