Yet another better/easier method:
This should set the default language to Korean on iTunes only:
defaults write -app iTunes AppleLanguages "(ko)"
On OSX with English as the default language, Korean letters will be broken on the iTunes and Mail and here is how to fix that:
defaults write $(mdls -name kMDItemCFBundleIdentifier -raw /Applications/iTunes.app) AppleLanguages "(ko, en)"
defaults write $(mdls -name kMDItemCFBundleIdentifier -raw /Applications/Mail.app) AppleLanguages "(ko, en)"