That tweet is exactly what I mean by hearsay. A few clips cherrypicked from thousands of hours of footage, showing a police officer speaking a few words in the second most popular language in the area is not evidence that they're undercover Chinese officers.
"Second most popular language in the area" is a gross distortion of reality. It would make zero sense for a local police officer to choose to yell at local protestors or local colleagues in a non-native language that carries significant stigma in HK.
It's not a "gross distortion of reality" to question a interpretation a clip in a way that you disagree with, especially since you lack the context as well. It doesn't change how little evidence there is that Chinese officers were secretly policing Hong Kong.
Depending on what figures you look at, English may be more popular. Fairly close these days, though.