I think there might not be a word specifically for this, but there may be a more general word that covers all case where you switch from one language to another during a conversation or in writing. I suspect that it is the case, because you encounter it in writing and in countries where people use more than one language.
For example:
"I am aware that ce n'est pas vraie, but I would still caution prudence." said Roberto to his friend Mathieu.