By default, the search will use english. to change it to use italian, change line 3 of the Yahoo/BOSS_include.php file:
$url = 'http://boss.yahooapis.com/ysearch/web/v1/'.urlencode($term).'?appid='.$appID.'&view=keyterms&sites='.urlencode($sites).'&abstract=long&count='.$count.'&format=xml&start='.$start;
to:
$url = 'http://boss.yahooapis.com/ysearch/web/v1/'.urlencode($term).'?appid='.$appID.'&view=keyterms&lang=it®ion=it&sites='.urlencode($sites).'&abstract=long&count='.$count.'&format=xml&start='.$start;