Document setlocale() threadsafety issue
"Fixes" //github.com/harfbuzz/harfbuzz/issues/1191
diff --git a/src/hb-common.cc b/src/hb-common.cc
index a9c62e8..16f8701 100644
--- a/src/hb-common.cc
+++ b/src/hb-common.cc
@@ -361,7 +361,14 @@
/**
* hb_language_get_default:
*
+ * Get default language from current locale.
*
+ * Note that the first time this function is called, it calls
+ * "setlocale (LC_CTYPE, nullptr)" to fetch current locale. The underlying
+ * setlocale function is, in many implementations, NOT threadsafe. To avoid
+ * problems, call this function once before multiple threads can call it.
+ * This function is only used from hb_buffer_guess_segment_properties() by
+ * HarfBuzz itself.
*
* Return value: (transfer none):
*
