Use LC_ALL instead of LANG to force locale to C

This is an explicit override that even works if one of the other LC_*
variables are set. The LC_ALL variable also needs to be exported.

update-po-files.sh

@@ -10,7 +10,7 @@
 #
 #####################################
 
-LANG=C
+export LC_ALL=C
 
 display_help() {
   echo "Usage: $0 [-k] [-p] [locale]"