>Perhaps whatever language they were using to think of the subject, or when they need a certain word.
>I don't really know what prompts the language switch.
>
>It appears that the speakers don't even notice they are switching languages.

Some hundred years ago over here, if you meant to do business, you had to speak all three main languages (serbian, german and hungarian). The conversation was customarily led in the language of whoever speaks first. And then there were other local languages - Romanian, Slovakian.