When marking "English" as translatable, Drupal will fail to locate a proper po file. That message is correct, however, there are double-escaping issues. Example:
Drupal core (8.0.0-beta1). File not found at <em class="placeholder">http://ftp.drupal.org/files/translations/8.x/drupal/drupal-8.0.0-beta1.en.po</em> nor at <em class="placeholder">translations://drupal-8.0.0-beta1.en.po</em>
To be determined
Probably fairly simple code, we're looking into it.
User interface changes
@akoe and me will work on it