Using pdfpages
you cannot import the TOC, but you can recreate it. But I don't think this is worth the effort in your case. Especially not, when you are writing both versions of the book. In this case I would consider \include
instead of \includepdf
. See the example below.
Anyway, assuming that you have only the PDF (not the LaTeX source), then it might be reasonable to recreate the TOC like this:
\documentclass[a4paper]{article}
\usepackage{pdfpages}
\usepackage{hyperref}
\begin{document}
\includepdf[
pages=-,
addtotoc={%
1, section, 1, aaa, sec:aaa,
2, section, 1, bbb, sec:bbb,
3, section, 1, ccc, sec:ccc
}%
]{dummy.pdf}
\end{document}
But since you are writing both versions of the book, and have the LaTeX source of both, the following approach is much better.
English version of your book. File content-en.tex
.
\section{English}
\lipsum
French version of your book. File content-fr.tex
.
\section{French}
\lipsum
And finally the main document, where you include both versions or just one or the other.
\documentclass{article}
\usepackage{lipsum}
\usepackage{hyperref}
\begin{document}
\include{content-en}
\include{content-fr}
\end{document}