AUCTeX recognizes french latex files by this code fragment.
\usepackage[frenchb]{babel}
This will install french style quoting via key ', but no font-lock support. To add this, you can put
(add-hook 'TeX-language-fr-hook
(lambda ()
(font-latex-add-quotes '("\\og" "\\fg{}")))))
in your init-file.
muede