\documentclass{article}

\usepackage[latin2]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[magyar]{babel}
\usepackage{array,delarray}

\begin{document}

\newcolumntype{L}{>{$}l<{$}}
\[  f(x)= \begin{array}\{{ l L }.
             0 & ha $x$ racionális,\\
             1 & ha $x$ irracionális.
          \end{array}  \]

\end{document}