[XeTeX] warnings with Polyglossia

[Yves Codet]

Hello.

If I typeset a document such as this one:
%%%%%%%%%%%%%%%%%%%%
\documentclass[10pt,a4paper]{book}
\usepackage{polyglossia}
\usepackage{fontspec}
\setdefaultlanguage{french}
\newfontfamily\frenchfont{Charis SIL}
\setmainfont{Charis SIL}

\begin{document}

\chapter{Chapitre}

Texte.

\end{document}
%%%%%%%%%%%%%%%%%%%%

for each chapter title I get a warning such as:

Package polyglossia Warning:  french at font is not defined
  in gloss-french.ldf on input line 10.

Should "frenchfont" be defined in another way than the above?

Another question is the following: "\newfontfamily\frenchfont{Charis  
SIL}" and "\setmainfont{Charis SIL}" seem to be redundant, but if I  
comment out "\setmainfont{Charis SIL}" the text is typeset with  
another font than Charis (I don't know which one). Are both commands  
necessary?

Thanks in advance.

Best wishes,

Yves