Hi,

if the multilanguage plugin is disabled using a third party SEF/multilingual extension, change the setting in the JSitemap config as shown in the image below choosing 'Show always dropdown'.
This will have the effect to show the multilanguage dropdown in the SEO dashboard.

After that ensure to set the paramter 'Enable SEF links for sitemap generation' to 'No', in this way you will have raw links with multilanguage to generate different sitemaps that will work:
http://www.motustriathlon.com/index.php?option=com_jmap&view=sitemap&format=xml&lang=it
http://www.motustriathlon.com/index.php?option=com_jmap&view=sitemap&format=xml&lang=en

Best regards

John Dagelmore