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You can enable foreign language input if you can find the appropriate keyboard in the Market or APK (non-Market app). Try searching the Market for Korean. Once you've downloaded the keyboard you want, go to Settings --> Language and Keyboard --> put a checkmark next to your new foreign language keyboard.
Then go to any input box (Notes, Messages, etc) and before you start typing, tap and long hold the input box. This will give you options, such as "Copy", 'Paste", and "Input method. Tap "Input method" and select the keyboard you want to use.
If by "display language" you mean changing all of the menus and everything else to another language, I'm not sure there's an easy way. You may have to change ROMs. I think it's like any other operating system. If you buy English Windows, you can't turn it into Korean Windows. You have to buy Korean Windows in the first place.
I have found apps that will let you pseudo-change your locale, but it doesn't turn everything into another language. This might be good enough for you without going through the hassle of rooting and changing ROMs. Try searching for a Korean one in the Market.