The language FA works wherever his/her seniority will allow them to...so it could be F,C or Y. In other words, it's not automatically the case that they'll be positioned in Economy.

As others have suggested, I'd just make sure to have "Limited English speaking ability" added to her PNR and hand write her a note saying the same thing adding in "can you please get get me the Korean speaking flight attendant?" which she can proactively present to the crew on board the aircraft.

The FC menu should be printed in both English and Korean, so assuming she in literate/has good eye site, she shouldn't have too much trouble selecting her food choices, etc.