\begin{center}
\begin{minipage}{0.7\textwidth}

\selectlanguage{english}

\chapter*{Abstract}

\blindtext

\end{minipage}
\end{center}

\blankpage

\begin{center}
\begin{minipage}{0.7\textwidth}

\selectlanguage{ngerman}

\chapter*{Zusammenfassung}

\begin{otherlanguage}{ngerman}

\blindtext

\end{otherlanguage}

\end{minipage}
\end{center}