build: add a test to check the Windows installer languages.

This way, we will avoid in the future any discrepancies between
languages set in the Windows installer and available translations (po
files in po-windows-installer/) because `make check`/`ninja test` will
fail.

build/windows/installer/lang/Makefile.am

@@ -71,3 +71,9 @@ clean-local:
 		echo "  RM       $$isl";		\
 		rm -f "$$isl";				\
 	done
+
+TESTS_ENVIRONMENT = \
+	chmod +x test-installer-langs.sh; \
+	GIMP_TESTING_ABS_TOP_SRCDIR=@abs_top_srcdir@
+
+TESTS = test-installer-langs.sh

build/windows/installer/lang/meson.build

@@ -59,3 +59,10 @@ foreach language : languages
     build_by_default: true,
   )
 endforeach
+
+test('windows-installer-langs',
+  find_program('test-installer-langs.sh'),
+  env: [
+    'GIMP_TESTING_ABS_TOP_SRCDIR=' + meson.source_root(),
+  ],
+  suite: 'build')

build/windows/installer/lang/test-installer-langs.sh

@@ -0,0 +1,21 @@
+#!/bin/sh
+
+# Make sure that the languages specified in the installer match the
+# translations present. This check step is necessary to not forget new
+# installer translations because we have a manual step.
+
+INSTALLER_LANGS=`grep -rI '^Name:.*MessagesFile' ${GIMP_TESTING_ABS_TOP_SRCDIR}/build/windows/installer/gimp3264.iss | \
+                 sed 's/^Name: *"\([a-zA-Z_]*\)".*$/\1/' | sort`
+# 'en' doesn't have a gettext file because it is the default.
+INSTALLER_LANGS=`echo "$INSTALLER_LANGS" | tr '\n\r' ' ' | sed 's/\<en\> //'`
+
+PO_LANGS=`ls ${GIMP_TESTING_ABS_TOP_SRCDIR}/po-windows-installer/*.po |sed 's%.*/po-windows-installer/\([a-zA-Z_]*\).po%\1%'`
+PO_LANGS=`echo "$PO_LANGS" | tr '\n\r' ' '`
+
+if [ "$PO_LANGS" = "$INSTALLER_LANGS" ]; then
+  exit 0
+else
+  echo "Error: languages listed in the Windows installer script do not match the .po files in po-windows-installer/."
+  echo "Please verify: build/windows/installer/gimp3264.iss"
+  exit 1
+fi