[API] Add HB_LANGUAGE_INVALID
diff --git a/src/hb-common.h b/src/hb-common.h
index a30587a..f0fe63a 100644
--- a/src/hb-common.h
+++ b/src/hb-common.h
@@ -114,6 +114,8 @@
 const char *
 hb_language_to_string (hb_language_t language);
 
+#define HB_LANGUAGE_INVALID ((hb_language_t) NULL)
+
 hb_language_t
 hb_language_get_default (void);