chore: insert Language header entry when merging from CPython doc

Makefile

@@ -84,11 +84,11 @@ endif
  if [ -f "$$PO" ];\
  then\
  case "$$POT" in\
- *whatsnew*) msgmerge --backup=off --force-po --no-fuzzy-matching -U "$$PO" "$$POT" ;;\
- *) msgmerge --backup=off --force-po -U "$$PO" "$$POT" ;;\
+ *whatsnew*) msgmerge --lang=$(LANGUAGE) --backup=off --force-po --no-fuzzy-matching -U "$$PO" "$$POT" ;;\
+ *) msgmerge --lang=$(LANGUAGE) --backup=off --force-po -U "$$PO" "$$POT" ;;\
  esac\
  else\
- msgcat -o "$$PO" "$$POT";\
+ msgcat --lang=$(LANGUAGE) -o "$$PO" "$$POT";\
  fi\
  done