Add checkLanguageCode util function

core/modules/utils/utils.js

@@ -162,6 +162,18 @@ exports.toTitleCase = function(str) {
 	return (str || "").replace(/(^|\s)\S/g, function(c) {return c.toUpperCase();});
 }
 
+/*
+Check whether a language code is supported by Intl
+*/
+exports.checkLanguageCode = lang => {
+	try {
+		"a".localeCompare("a", lang);
+		return true;
+	} catch(e) {
+		return false;
+	}
+}
+
 /*
 Find the line break preceding a given position in a string
 Returns position immediately after that line break, or the start of the string