What is the main reason for having FA? Safety.
If they cannot communicate in an emergency with the SLF safety is compromised.
Otherwise I am the most stupid SLF on earth.
You'll find that all emergency instructions on board are illustrated.
If the PAX are too thick to understand that, then that is their problem. Do you expect two FAs to speak every language on the planet ??
Think about the logical conclusion to this argument. BA flight LHR-PEK, the rostered FA who speaks Chinese goes sick at the last minute. What do you do, delay the flight as it has 50/50 English/Chinese speakers as PAX ????