You need to use == instead of = and if the code is inside a function, you need a global line or you can pass the variable in.
function my_function()
{
global $linguaggio;
if ($linguaggio=="italiano") {include("italiano.php");}
if ($linguaggio=="english") {include("english.php");}
}
or
function my_function($linguaggio)
{
if ($linguaggio=="italiano") {include("italiano.php");}
if ($linguaggio=="english") {include("english.php");}
}