I agree that this is often the case. But it is also true that when you know more than one language well you often find that words in one language are more expressive than in the other. So it becomes convenient to mix the languages in conversation by chosing a single word in one language that would be an entire phrase in the other.
As I mentioned in https://news.ycombinator.com/item?id=14830685, there is a major difference between deliberately mixing languages when talking to a fellow bilingual as an adult, and the accidental mixing of languages when talking to anyone that kids do because they aren't fluent enough.
Of course, it can in some situations be applied to both adults and kids. My wife speaks Tagalog and English, which combined is the official language of Taglish. Sometimes she does not know the actual word for something in Tagalog so she substitutes it with English. This can fir example involve words for colours. More often though, the substitution is purely to emphasize.
For me, as we only speak Swedish in Sweden, I do not have this habit of mixing and substition. That being said, my English is of curse much worse then my Swedish...
I'm hoping that our kids can speak all three languages natively, and I'm sure they will naturally mix it in between them self. And that in other settings, that they uses the current locale with out thinking about it.
I'm also hoping the knowledge of many languages makes them more susceptible to even more languages and cultures. :)
