Use g_get_language_names in rsvg_cond_parse_system_language
Use g_get_language_names in rsvg_cond_parse_system_language

Using the first element keeps the existing behavior but avoids a
bunch of platform specific code and removes the need to allocate
and free the string each time.

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg