I agree,
Having fluency in English is probably the most important skill as they already have their native language to offer. Carriers like CX hire crew from all over Asia with different languages but they all have one thing in common, English. Of course another Asian or European language would be beneficial but depends on the airline they choose to work for. Do they wish to work for an airline who solely wants them for their language skills (in this case probably only doing Thai flights.
As far as Eurpoean languages go I would suggest German, French, Dutch or Spanish.