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:
1, section, 1, aaa, sec:aaa,
2, section, 1, bbb, sec:bbb,
3, section, 1, ccc, sec:ccc
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
French version of your book. File
And finally the main document, where you include both versions or just one or the other.