메리 크리스마스!

메리 크리스마스: Me-ri Keu-ri-seu-ma-seu
In Afrikaans: Geseende Kersfees!
In Chinese: 聖誕快樂 Shèngdàn kuàilè!
In Taiwanese: Sèng-tàn khòai-lo̍k!

In Korean, you could also say 성탄절 (Seong-tan-jeol) for Christmas. This name is derived from the Chinese characters: 聖誕節 (Shèngdànjié), although I haven't heard anyone use this word in the context of a seasonal greeting.

Me and the Christmas "Tree" in Changwon.

Merry Christmas from 켈리포니아 (Kel-li-po-ni-a)! It's good to be home.

Here is a cute video of Koreans singing Christmas carols and dancing in a train station.