\documentclass[12pt,a4paper]{report}
\usepackage{fontspec}
\usepackage{polyglossia}
\setdefaultlanguage{french}
\setotherlanguage[variant=ancient]{greek}
\setmainfont{Charis SIL}
\newfontfamily\greekfont{Lucida Grande}
\newcommand{\grk}[1]{{\begin{greek}#1\end{greek}}}

\begin{document}

\input{./ζαθεος}

\end{document}