Code switching is the practice of moving back and forth between two languages, or between two dialects or registers of the same language. Also called code-mixing and style shifting.
Five Reasons Why People Code-Switch
1) Our lizard brains take over.
2) We want to fit in.
3) We want to get something.
4) We want to say something in secret.
5) It helps us convey a thought.
Linguistics 