Well it really depends on the game but I think they treat the western market as the lower priority one, a lot of these games come out in China first for example, and then they do the western stuff. They could do it in parallel but I guess but they need to find a company to do their work overseas to delegate all the stuff like localization, marketing, renting servers etc so it makes more sense to do it one region at a time. The translation also takes a fair bit of time quite often cause there's a lot of text in mmos, and they usually won't have many people working on it, plus I'd assume korean translators aren't that common.