I booked my mom on UA F TPAC, but she doesn't speak English very well
I travel on the SFO-ICN flight a lot. There are always Korean speaking FA's and the 1st class menu is in English / Korean / Chinese.
I think if you call UA res they can put a notation in the PNR so the crew is aware in advance.
I am on the flight this coming Saturday, so I'll ask re the FA's and I do believe there is a US Customs form in Korean too ... I'll look and pm you