WebUIs: set the lang attribute based on the chosen language

Signed-off-by: Nicola Murino <nicola.murino@gmail.com>

templates/common/base.html

@@ -178,6 +178,7 @@
     };
 
     const renderI18n = () => {
+        document.documentElement.setAttribute('lang', i18next.resolvedLanguage);
         $('title').text('{{.Branding.Name}} - '+$.t('{{.Title}}'));
         $('body').localize();
         let select2elements = [].slice.call(document.querySelectorAll('[data-control="i18n-select2"]'));