According to Henri Choi (at his excellent and often in Swedish blog Underbart Landet) the most internationally translated Korean author is Yi Mun-yol.
The Korean writer most represented in terms of languages is Yi Mun-yol, with over 50 works translated into a total of 16 different languages, according to a report released by the Korean Literature Translation Institute at the end of 2008. 48 works by the poet Ko Un have appeared in 15 languages, while translations of 37 books by Hwang Sok-yong have been published in 11 languages.
I sometimes gripe about who is chosen for translation, but I don’t think there is any way to argue that Yi has not been the best choice for translation in the past. I hear that Yi Mun-yol is working on a new novel, after many years of silence, so there may be another translation coming down the pike.
This report that Mr. Choi talks about sounds interesting, and I’ll have to get a copy when I get back to Seoul.