Communication means so much more than language

I had an interview recently with a French gentleman who works at Sciences Po, the prestigious French social science institute in Paris, on the subject of Chinese students coming to study in France.

The interview was in English as we both assumed that we weren't able to speak each other's languages. When he talked about the Chinese educational system, he suddenly and almost unconsciously switched to Chinese as if he was more comfortable with speaking Chinese than English.

At the moment I realized that we, the Chinese and the French, perhaps know each other better than we think. So why didn't we bypass the third language in the first place?