It uses locale information to detect default language. It's used by
hb_shape() whenever language is not set on the buffer.
Not sure how to properly test it in the test suite. Tested by observing
that with DejaVu Sans we select the proper local glyph version for U+431
under Serbian locale. See http://www.pango.org/ScriptGallery
Note that the static variable has to be a global static, as gcc
implements local statics differently and that would require linking
to libstdc++, which we don't want.
The Win32 definitions for LONG, ULONG, etc conflicts with
hb-open-type.h. Avoid that by making sure hb-object-private.h
and hb-open-type.h are not included in the same compilation unit.