spoken language question?

I have just watched this on Danish TV. Brando and Schneider speak both French and English at different times throughout the movie. In some scenes, one character might be speaking French and another English. And they switch mid-conversation sometimes.
For example, in the scene where Brando talks with his wife's ex-lover, the whole thing is in French until the end when Brando says in English "I wonder what she ever saw in you".
While this might seem unrealistic to some, believe me, I am an Australian living in Denmark and I can promise you it does happen... drifting in and out of languages. It's not uncommon to be speaking a second language and when an impulsive thought comes over you it is very easy to lapse in to your mother tongue.
Obviously English is my first language, Danish my second, and French a long way behind in third ... and what I watched was entirely subtitled in Danish so somewhere along the way I got it all.

yes it was supposed to be in French, his character Paul was an american who moved to Paris, and therefore learned and could speak fluent French.

In my opinion the greatest acting performance of all time...