patch 8.2.0636: :messages does not show the maintainer when $LANG is unset

Problem:    :messages does not show the maintainer when $LANG is unset.
Solution:   Call get_mess_lang() if available. (closes #5978)
diff --git a/src/message.c b/src/message.c
index fbddfad..04dc401 100644
--- a/src/message.c
+++ b/src/message.c
@@ -1035,7 +1035,11 @@
 
     if (p == first_msg_hist)
     {
+#ifdef FEAT_MULTI_LANG
+	s = get_mess_lang();
+#else
 	s = mch_getenv((char_u *)"LANG");
+#endif
 	if (s != NULL && *s != NUL)
 	    // The next comment is extracted by xgettext and put in po file for
 	    // translators to read.