The three most populous languages are Mandarin by population of China, Spanish and English. English is generally considered the universal language of Aviation.
in the function of their duties The PLAAF like any other air force are not limited to Kill kill kill. They have to interact with other aviators from across the world. When doing interceptions of American, Japanese, Korean, Russian, Malaysian, Pilipino, Pakistani, Hindu... or any other nationality. They need to be able to Warn off, Threaten, cajol, plead, Aid, address, command whatever the situation demands. To do that you need both sides to have a common frame of reference. The ability to exchange numbers and technical terminology in order to plot course redirect or escort. As such English is the universal language. A pilot must be able to speak it and navigate with it. As simply shouting into the mic in Mandarin won’t help if the guy on the receiver doesn’t understand a word of it. And speaking High school grade French doesn’t do a lick of good if the conversation is of a technical matter.
If the PLAAF wants to intercept a Boeing 737 pilot simply trying to communicate with close flybys and sharp maneuvers doesn’t help anything it simply makes them look like daredevils or worse like they intend to kill the bird.
Being able to at least make claim over the radio in a common language can keep it from being misconstrued as overtly hostile.