This is used as the language index in
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_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.