Whether I use the french language with babel or polyglossia, irrespective of the engine, the back references for the footnotes are missed.
\documentclass{article}
\usepackage{ifpdf,ifxetex,ifluatex}
\ifluatex
\usepackage{fontspec}
\setmainfont{TeX Gyre Termes}
\setmainfont{GentiumPlus}
\newfontfamily\greekfont{Linux Libertine O}
\newfontfamily\russianfont{Linux Libertine O}
\newfontfamily\hindifont{Siddhanta}
\usepackage{polyglossia}
\setmainlanguage{french}
%\setmainlanguage{english}
%\setmainlanguage{czech}
%\setmainlanguage{greek}
%\setmainlanguage{russian}
%\setmainlanguage{german}
%\setmainlanguage{hindi}
%\setotherlanguage{english}
\else
\ifxetex
\usepackage[T1]{fontenc}
\usepackage{fontspec}
\setmainfont{TeX Gyre Termes}
%\setmainfont{GentiumPlus}
%\newfontfamily\greekfont{Linux Libertine O}
%\newfontfamily\russianfont{Linux Libertine O}
%\newfontfamily\hindifont{Siddhanta}
\usepackage{polyglossia}
%\setdefaultlanguage{french}
\setdefaultlanguage{english}
%\setotherlanguage{czech}
%\setotherlanguage{german}
%\setotherlanguage{greek}
%\setotherlanguage{russian}
%\setotherlanguage{hindi}
%\setotherlanguage{english}
%\usepackage[french]{babel}
%\usepackage[babel,french=quotes]{csquotes}
\else
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
%\usepackage[french]{babel}
\usepackage[french=quotes]{csquotes}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\fi\fi
\usepackage{footnotebackref}
\begin{document}
Some text with a footnote\footnote{The first footnote.}\\
Text with the second footnote\footnote[4]{The second footnote.}
\end{document}