It is a common belief that when someone alternates between two or more languages in conversation, a phenomenon known as code-switching, it is because they lack proficiency in one of the languages. I beg to differ.
When I'm relaxed and talking to friends or family members who speak French and English, I often switch between the two languages because, well, I don't have to think so much. I (just like other code-switchers) often just use the first word that comes to me, be it in French or in English. If I start a sentance in Engish for example, and I can't put my finger on the right word for what I want to say, I just say it in French.
Although it may feel like the easy option, when fluent speakers of two or more languages can simultaneously accessing information from both.
There is a fun side to code-switching too. Speaking Franglais, for example can be rather amusing. Deliberately alternating between French and English, mixing both languages together, whilst respecting the rules or syntax and grammar can be like a game. One can play with words.
Code-switching can actually enable people to be more expressive.