sant0sk1

No, line 588 has a single curly brace on it: }. The preceding two lines are commented out, and it appears that the author just failed to also comment out that line.

After deleting line 588, the working function looks like this:

function wppa_qtrans($output, $lang = '') {
  if ($lang == '') {
    $output = __($output);
//  if (function_exists('qtrans_useCurrentLanguageIfNotFoundUseDefaultLanguage')) {
//  $output = qtrans_useCurrentLanguageIfNotFoundUseDefaultLanguage($output);
  } else {
    if (function_exists('qtrans_use')) {
      $output = qtrans_use($lang, $output, false);
    }
  }
  return $output;
}

I think Disqus needs to write a fix for this, but for now you can simply delete that entire script block from the plugin and your feed will validate again.

The lines to remove are 278-292 in version 2.11.4349 of the plugin.