Fix HTML `lang` attribute using the `defaultLang`

Themes need `lang="{function.localeToHTML, userLang, defaultLang}"` in
their header.tpl file

public/src/modules/helpers.js

@@ -176,7 +176,8 @@
 		}).join('');
 	};
 
-	helpers.localeToHTML = function (locale) {
+	helpers.localeToHTML = function (locale, fallback) {
+		locale = locale || fallback || 'en-GB';
 		return locale.replace('_', '-');
 	};
 

src/middleware/header.js

@@ -133,6 +133,7 @@ module.exports = function (middleware) {
 				templateValues.customJS = templateValues.useCustomJS ? meta.config.customJS : '';
 				templateValues.maintenanceHeader = parseInt(meta.config.maintenanceMode, 10) === 1 && !results.isAdmin;
 				templateValues.defaultLang = meta.config.defaultLang || 'en-GB';
+				templateValues.userLang = res.locals.config.userLang;
 				templateValues.privateUserInfo = parseInt(meta.config.privateUserInfo, 10) === 1;
 				templateValues.privateTagListing = parseInt(meta.config.privateTagListing, 10) === 1;