Allow editor language to be chosen in editor settings

This gets stored in localStorage of the browser which is not
ideal. This is because we load language catalogs before we
load user preferences - so if this was stored in the runtime,
the editor wouldn't know the user's preference until it was
too late to apply it.

This is likely good enough for now - may need to do something
more convoluted later on.

packages/node_modules/@node-red/util/lib/i18n.js

@@ -50,10 +50,21 @@ function registerMessageCatalogs(catalogs) {
 function registerMessageCatalog(namespace,dir,file) {
     return initPromise.then(function() {
         return new Promise((resolve,reject) => {
-            resourceMap[namespace] = { basedir:dir, file:file};
-            i18n.loadNamespaces(namespace,function() {
-                resolve();
-            });
+            resourceMap[namespace] = { basedir:dir, file:file, lngs: []};
+            fs.readdir(dir,function(err, files) {
+                if (err) {
+                    resolve();
+                } else {
+                    files.forEach(function(f) {
+                        if (fs.existsSync(path.join(dir,f,file))) {
+                            resourceMap[namespace].lngs.push(f);
+                        }
+                    });
+                    i18n.loadNamespaces(namespace,function() {
+                        resolve();
+                    });
+                }
+            })
         });
     });
 }
@@ -163,11 +174,24 @@ function getCatalog(namespace,lang) {
     return result;
 }
 
+/**
+ * Gets a list of languages a given catalog is available in.
+ * @name availableLanguages
+ * @function
+ * @memberof @node-red/util_i18n
+ */
+function availableLanguages(namespace) {
+    if (resourceMap.hasOwnProperty(namespace)) {
+        return resourceMap[namespace].lngs
+    }
+}
+
 var obj = module.exports = {
     init: init,
     registerMessageCatalog: registerMessageCatalog,
     registerMessageCatalogs: registerMessageCatalogs,
     catalog: getCatalog,
+    availableLanguages: availableLanguages,
     /**
      * The underlying i18n library for when direct access is really needed
      */