Hi,
The problem is related to .html extension. I can see the following link
http://www.sigilplast.it/fr.htmlPlease try to add the following rules in your .htaccess file after rewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_URI} !index.php
RewriteCond %{REQUEST_URI} !(.*)/$
RewriteRule ^(.*)$
http://sigilplast.it/$1/ [L,R=301]