`preferred_locale` might be a JSON string, not an object

src/utils.js

@@ -459,12 +459,12 @@
     utils.getLocale = function (preferred_locale, isSupportedByLibrary) {
         if (isSupportedByLibrary(preferred_locale)) {
             return preferred_locale;
-        } else if (_.isObject(preferred_locale)) {
-            try {
-                return preferred_locale.locale_data.converse[""].lang;
-            } catch (e) {
-                console.log(e);
-            }
+        }
+        try {
+            var obj = window.JSON.parse(preferred_locale);
+            return obj.locale_data.converse[""].lang;
+        } catch (e) {
+            console.log(e);
         }
         return utils.detectLocale(isSupportedByLibrary) || 'en';
     };