I cannot comment on CX flight attendants, since my CX flying experience is somewhat limited.
However, I have been on a few Air China flights(EU-China-Asia) and almost all of the flight attendants spoke nearly-perfect English. Compare this to 10 or 15 years ago when I couldn't understand even 20% of what they were saying(and vice versa).
Oh well, at least CX isn't as bad as SQ when it comes to English language skills, not yet anyway.
I had a SQ FA ask me if I wanted an extra "pee-low" on a recent SYD-SIN flight in C. I didn't want to embarrass her so I replied "yes, I'd like an extra pee-low, please." (she meant pillow, I hoped)
IMHO, as long as the job gets done, I couldn't care less how good the FA's English is.
It's not like she is negotiating a nuclear arms reduction treaty.