그렇게 좋은 솔루션은 아닌거 같긴 한데... 다음과 같은 방법도 있습니다. 

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\documentclass{oblivoir}

\usepackage{pgfkeys,ifthen}

\let\SetValue\pgfkeyssetvalue

\SetValue{language}{both}

\newcommand{\ProcessData}{

\ifthenelse{\equal{\pgfkeysvalueof{language}}{both}}{

\pgfkeysvalueof{korean}

\pgfkeysvalueof{english}}

{}

\ifthenelse{\equal{\pgfkeysvalueof{language}}{korean}}{

\pgfkeysvalueof{korean}}

{}

\ifthenelse{\equal{\pgfkeysvalueof{language}}{english}}{

\pgfkeysvalueof{english}}

{}

}

\begin{document}

\SetValue{korean}{안녕하세요}

\SetValue{english}{Hello}

\SetValue{language}{both}

\ProcessData

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\SetValue{language}{english}

\ProcessData

----------------------------------------------------------

\SetValue{language}{korean}

\ProcessData

\SetValue{korean}{만나서 반가워요.}

\SetValue{english}{Nice to meet you}

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\SetValue{language}{both}

\ProcessData

----------------------------------------------------------

\SetValue{language}{english}

\ProcessData

----------------------------------------------------------

\SetValue{language}{korean}

\ProcessData

\end{document}