documentclass{article}
usepackage{polyglossia}
setmainlanguage{korean}
setmainfont{UnBatang}

begin{document}
한국
end{document}

You cannot set field spec in a node of type glue.
newpage ...k @nobreakfalse everypar {}fi par
                                              ifdim prevdepth >z@ vs...

Package polyglossia Warning: No hyphenation patterns were loaded for `orean'
(polyglossia)   I will use language=l@nohyphenation instead on input line 6.