Issue #12776: Underscored .isl langs (pt_BR, zh_CN and zh_TW) lost translation on Installer.

This is again a change in gettext 0.23 whose release notes say:

> The value of the xml:lang attribute, inserted by msgfmt, now conforms to W3C standards.

Our xpath selection now recognizes both the old and new (proper) format
so that it works both with older and new versions of msgfmt.

build/windows/installer/lang/meson.build

@@ -58,11 +58,14 @@ languages = [
 # Then, we generate the .isl file for each language using some xsltproc magic
 foreach language : languages
   lang_code = language.get('code')
+  # Since gettext 0.23, outputted xml:lang by msgfmt conforms to W3C
+  # standard. Try both with underscore and hyphens.
+  web_lang_code = lang_code.replace('_', '-')
 
   if 'default' in language
     lang_check = 'value[not(@xml:lang)]'
   else
-    lang_check = 'value[lang(\'@0@\')]'.format(lang_code)
+    lang_check = 'value[lang(\'@0@\')] | value[lang(\'@1@\')]'.format(lang_code, web_lang_code)
   endif
 
   gimp_ms_installer_lang_xsl = configure_file(