\begin{center}
\begin{minipage}{0.7\textwidth}
\selectlanguage{english}
\chapter*{Abstract}
TODO
\end{minipage}
\end{center}
\blankpage
\begin{center}
\begin{minipage}{0.7\textwidth}
\selectlanguage{ngerman}
\chapter*{Zusammenfassung}
\begin{otherlanguage}{ngerman}
TODO
\end{otherlanguage}
\end{minipage}
\end{center}