patch 8.2.4420: menu translations are inconsistent

Problem:    Menu translations are inconsistent.
Solution:   Add a Makefile to convert between encodings. (Ada (Haowen) Yu,
            closes #9801)
diff --git a/runtime/lang/menu_sl_si.latin2.vim b/runtime/lang/menu_sl_si.latin2.vim
index cf4b10b..99f2aa1 100644
--- a/runtime/lang/menu_sl_si.latin2.vim
+++ b/runtime/lang/menu_sl_si.latin2.vim
@@ -3,6 +3,7 @@
 " Originally By:	Mojca Miklavec <mojca.miklavec.lists@gmail.com>
 " Last Change:		2020 Apr 23
 " vim:set foldmethod=marker tabstop=8:
+" Generated from menu_sl_si.utf-8.vim, DO NOT EDIT
 
 " TODO: add/check all '&'s