I have no knowledge of Korean, but I am quite familiar with postal systems. Korea Post will accept both, as it's the usual practice, but because it's inbound mail, they would prefer the address in Korean. Your friend likely did the opposite for the same reason. Properly addressed letters are more often delivered than returned, so it makes sense. It wouldn't be strange to have the return address written the opposite way, since, besides the name of the country, the return address is not really going to be read by Korea Post Remember to write "Rep. of Korea" or similar in a familiar local language, otherwise, your post office might not know where to send the letter.