I think it is just an automatic response to a caucasian face, whether they speak Chinese or not.

My whole family speaks Mandarin and Taiwanese so it is more natural for me to speak Chinese with a Chinese speaking stranger. However 4 out of 5 FAs will respond in English, in which case I just go along with that.

It's not very nice for my wife when we are travelling together and the FAs assume that both of us speak English. My wife then gets very uncomfortable because there is a stigma in Taiwan that lack of English ability = lack of general intelligence.