I have to agree with all the previous posts… there are more mandarin speakers than Korean or Japanese.
You not only have to think about the current state of things… china will keep on growing and improving their design capabilities.
Still… it all depends where you want to work, or who you want to work with. I hear that in Japan is close to impossible to get a job if you don’t speak Japanese… so… if you want to go to Japan, learn Japanese.
Cheers!