PangoOTDEFAULT_LANGUAGE

This is used as the language index in pango_ot_info_find_feature() when the default language system of the script is desired.

It is also returned by pango_ot_info_find_language() if the requested language is not found, or the requested language tag was PANGO_OT_TAG_DEFAULT_LANGUAGE. The end result is that one can always call pango_ot_tag_from_language() followed by pango_ot_info_find_language() and pass the result to pango_ot_info_find_feature() without having to worry about falling back to default language system explicitly.