added missing translation string in extension manager

lib/plugins/extension/lang/en/lang.php

@@ -96,4 +96,6 @@ $lang['nopluginperms']                = 'Plugin directory is not writable';
 $lang['git']                          = 'This extension was installed via git, you may not want to update it here.';
 
 $lang['install_url']                  = 'Install from URL:';
-$lang['install_upload']               = 'Upload Extension:';
\ No newline at end of file
+$lang['install_upload']               = 'Upload Extension:';
+
+$lang['repo_error']                   = 'The plugin repository could not be contacted. Make sure your server is allowed to contact www.dokuwiki.org and check your proxy settings.';
\ No newline at end of file