Currently working on a new version of the plugin, and testing it alongside Polylang too, I see these 'language' pages appear. This development version handles languages quite differently but it might also depend on the Polylang settings...
Please install the Development version from http://wordpress.org/extend/plugins/xml-sitemap-feed/developers/ (download zip and upload via FTP, overwriting the current version files -- or uninstall the current version first and then install the zip via the WP back-end)
Please report any remaining issues after that :)