% !TeX document-id = {09f2812a-13fc-4c92-9510-5b529c8eef19}
% !TeX TXS-program:compile = txs:///xelatex/[--shell-escape]
\documentclass{article}
%%  fonts and special characters
%\usepackage[oldstyle]{libertine}% public domain font % \setmainfont unnecessary
\usepackage{ifxetex}% load different packages with/without XeLaTex
\ifxetex
  \usepackage{fontspec}% provides \setmainfont
  \setmainfont[Numbers=OldStyle,Ligatures=Common,Mapping=tex-text]{Linux Libertine O}% public domain font % use libertine package instead
\else  % not needed with utf8 based engines
  \usepackage[utf8]{inputenc}
  \usepackage[T1]{fontenc}
  \usepackage{lmodern}
\fi%
\usepackage[french]{babel}
\usepackage[babel]{csquotes}%\endquote,\textquote, \blockquote,\foreignquote
%%
\usepackage{lipsum}
%\usepackage[oldstyle]{libertine}% public domain font % \setmainfont unnecessary
\begin{document}
\section*{Testing fonts}
\lipsum
\end{document}