templates: Filter C.UTF-8 locale for translation

In addition to C locale there is also C.UTF-8 locale now. Filter that as
well, by using ${grub_lang}, which contains a stripped value.
This fixes the following message and resulting boot failure:

    error: file `/boot/grub/locale/C.gmo' not found.

Signed-off-by: Christian Hesse <mail@eworm.de>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

util/grub.d/00_header.in

@@ -191,7 +191,7 @@ EOF
 EOF
 
 # Gettext variables and module
-if [ "x${LANG}" != "xC" ] && [ "x${LANG}" != "xPOSIX" ] && [ "x${LANG}" != "x" ]; then
+if [ "x${grub_lang}" != "xC" ] && [ "x${LANG}" != "xPOSIX" ] && [ "x${LANG}" != "x" ]; then
   cat << EOF
   set locale_dir=\$prefix/locale
   set lang=${grub_lang}