To add to the information that Troy just told you, you can change the system language anytime to any one that you need---Mac OS is fully Unicode compliant and localized to most languages.
When you start your computer for the first time, it will ask which language you prefer, simply select English and all system menus, help files, and Apple software will be in English. You could also, for example, buy Aperture (the photo management software) in Korea, and when you install it, you just pick English as the install language. All Apple software is also Unicode, so it's just a matter of choosing your language when you install it (or it will install in the language of the system, but can be switched anytime). For Apple software, the only thing that is different in each country (as far as I know) is the box---the software is all the same---the install disc comes in all languages.
Also, whether you purchase from Apple online or in an Apple retail store, they will install an English keyboard, or in the case of the Apple store, they will swap out the Korean keyboard for an English one (at least they do in Japan, while you wait).