Format abstracts as unnumbered chapters

This just makes abstract include a chapter named with the command \abstractname, which is provided by babel and is re-defined for each language.

The lipsum package and \lipsum command are used only to provide some dummy text.

\documentclass{book}
\usepackage{lipsum}
\usepackage[english,french]{babel}
%% START HERE %%
\def\abstract{
  \chapter*{\abstractname} % start chapter
  \addcontentsline{toc}{chapter}{\abstractname} % table of contents line
  \markboth{\MakeUppercase{\abstractname}}{} % header mark
}
\def\endabstract{\clearpage}
%% END HERE %%
\begin{document}
\selectlanguage{english}
\tableofcontents
\begin{abstract}
\lipsum[1-20]
\end{abstract}
\selectlanguage{french}
\begin{abstract}
\lipsum[1-20]
\end{abstract}
\end{document}