Our helper runs as root and doesn't have the user's environment
(including $LANG). This defaults to one without UTF-8.
Without a locale with UTF-8
QFileInfo::exists("/home/david/wallpapers/crepé.png") returns false and
we fail to set a wallpaper.
By forcing the locale we fix that. It doesn't matter that we're using C,
as we don't have any UI related code in the helper.