documentclass{article}

usepackage{ifpdf,ifxetex,ifluatex}

ifluatex
usepackage{fontspec}
setmainfont{TeX Gyre Termes}
setmainfont{GentiumPlus}
newfontfamilygreekfont{Linux Libertine O}
newfontfamilyrussianfont{Linux Libertine O}
newfontfamilyhindifont{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}
%newfontfamilygreekfont{Linux Libertine O}
%newfontfamilyrussianfont{Linux Libertine O}
%newfontfamilyhindifont{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}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
fifi

usepackage{footnotebackref}
begin{document}
Some text with a footnotefootnote{The first footnote.}\
Text with the second footnotefootnote[4]{The second footnote.}
end{document}

documentclass{article}
usepackage{fontspec}
usepackage[french]{babel}
StandardFootnotes


usepackage{footnotebackref}

begin{document}
Some text with a footnotefootnote{The first footnote.}

Text with the second footnotefootnote[4]{The second footnote.}
end{document}