Subject behavior is absolutely improper, and I have always reminded me of not committing the same error myself.  I also told my kids to avoid such awkwardness when they started to receive education in America.  Even they are now speaking and thinking with English everyday, and either of them marries a Caucasian American, still they are speaking "pure" Mandarin with me, not to mingle with English as possible as they can. 

Attached below is an interesting report with caricatures, also serving a friendly dissuasion, towards those smatterers with such a bad way of speaking.

https://user.guancha.cn/main/content?id=107115&page=1