[FIX] Adding 'lang' tag (#16375)

client/startup/i18n.js

@@ -21,7 +21,7 @@ Meteor.startup(() => {
 		const regex = /([a-z]{2,3})-([a-z]{2,4})/;
 		const matches = regex.exec(language);
 		if (matches) {
-			return `${ matches[1] }-${ matches[2].toUpperCase() }`;
+			return `${ matches[1] }-${ matches[2].toLowerCase() }`;
 		}
 
 		return language;
@@ -56,8 +56,9 @@ Meteor.startup(() => {
 		if (!language) {
 			return;
 		}
-
 		document.documentElement.classList[isRtl(language) ? 'add' : 'remove']('rtl');
+		document.querySelector('html').lang = language;
+
 		TAPi18n.setLanguage(language);
 		loadMomentLocale(language)
 			.then((locale) => moment.locale(locale))