Jason Kottke is talking about codeswitching, a phenomenon where several multi-lingual people will switch between languages in a single conversation. Having had some first and second-hand experience of this, I posted the following in the comments:

quoteI spent several years in Europe, working with a friend with a very multi-cultural background. He's Armenian, born in Lebanon, was raised mostly in Cyprus, went to both English and French schools while there, and married a German.

He's very adept at languages, and as of last count, speaks Armenian, Arabic, Turkish, Greek, French, English, and German at least conversationally, if not fluently. His parents settled in Quebec, and I had the opportunity to overhear several calls home.

He, his siblings, and his parents would switch back and forth between Armenian, Arabic, English, and French mostly, in a single conversation. He explained that it usually had to do with whatever expression fit the circumstance, or in which language the joke was funniest. It was quite entertaining. smile endquote