The logic for this is already in gtk_get_default_language() (except that there's no need to special handle '.' and '@'.  I've fixed pango_language_from_string to do that already.)

The most non intrusive way is to just go on and add pango_get_default_language(), and leave it to the user to set context language to that of get_default_language.  

Created attachment 67605 [details] [review]
my patch

I wrote a patch for this bug. I added pango_get_default_language( ) in this patch.

Thanks LingNing.  Can you update the patch to include documentation (copy from gtk+ and adapt), and hook it into pango.def and docs/pango-sections.txt too?

ok, I will do it.
:)

Created attachment 67610 [details] [review]
modified patch

add the documentation , and add it into pango.def and docs/pango-sections.txt.

Committed after renaming to pango_language_get_default() and docs improvement.

2006-12-06  Behdad Esfahbod  <behdad@gnome.org>

        Bug 333982 – Fallback to $LANG whenever NULL PangoLanguage is used 
        Patch from LingNing Zhang

        * docs/pango-sections.txt:
        * docs/tmpl/utils.sgml:
        * pango/pango-context.c:
        * pango/pango-types.h:
        * pango/pango-utils.c (_pango_get_lc_ctype),
        (pango_language_get_default):
        * pango/pango.def:
        New public function pango_language_get_default().  Note that, this
        does not make Pango fallback to the default language automatically,
        but the user can use this function to set the default language of the
        locale on a context:

          pango_context_set_language (context, pango_language_get_default());