Setting language on export to 'und' if not defined

h5p.classes.php

@@ -1251,7 +1251,9 @@ Class H5PExport {
     // Build h5p.json
     $h5pJson = array (
       'title' => $content['title'],
-      'language' => $content['language'],
+      // TODO - stop using 'und', this is not the preferred way. 
+      // Either remove language from the json if not existing, or use "language": null 
+      'language' => isset($content['language']) ? $content['language'] : 'und',
       'mainLibrary' => $content['library']['name'],
       'embedTypes' => $embedTypes,
     );