I'm glad you asked. The NLP nerd in the past would start a lengthy textbook definition of that. But to make things simple. When people live in places where speaking multiple languages is a common practice, such as in Montreal, Quebec, les gens parlent en francais et anglais en meme temp. This is fine but most Natural Language Processing systems aren't equipped to deal with this, hence they need a codeswitching language identification preprocessor.

Decodeswitch was a codeswitching language detection machine learning model that I did some years ago. It combines subword language model with Conditional Random Field to produce a fast yet sufficiently accurate prediction. You can read more about it in the full webpage below.