Hide menu item in certain condition

In your actual code the function to add the script is never executed. You can put the function add_my_script() wherever you want but you need to hook it into a WordPress event. The best event for this case is the wp_enqueue_scripts:

add_action('wp_enqueue_scripts', 'add_my_script');
function add_my_script() {
    wp_enqueue_script(
        'hide-menu', // name your script so that you can attach other scripts and de-register, etc.
         get_template_directory_uri() . '/js/hide-menu.js', // this is the location of your script file
         array('jquery') // this array lists the scripts upon which your script depends
    );
}

Your javascript code is also wrong. According to jQuery selectors, you are trying to select all the <a> elements with a href attribute that contain the substring /it/noleggio-lungo-termine, but such elements doesn’t exist in your HTML when the laguage is not italian. The following code simply works to hide the li element with id=menu-item-9452:

jQuery("li#menu-item-9452").hide();

Read jQuery selectors documentation and use a valid selector.

To hide that element for certain languages, you will have to get access to the current language. I see that you are using qTranslate. A possible solution:

add_action('wp_enqueue_scripts', 'add_my_script');
function add_my_script() {

    wp_enqueue_script(
        'hide-menu', // name your script so that you can attach other scripts and de-register, etc.
         get_template_directory_uri() . '/js/hide-menu.js', // this is the location of your script file
         array('jquery') // this array lists the scripts upon which your script depends
    );

    $scriptData = array(
         'lang' => qtrans_getLanguage(),
    );
    wp_localize_script('hide-menu','script_data', $scriptData);
}

Ant then in hide-menu.js:

if( script_data.lang != 'it' ) {
    jQuery("li#menu-item-9452").hide();
}

Also you can hide that item by CSS. For example, in the header.php of your theme you can put this code between <head> and </head>:

<?php

 if( qtrans_getLanguage() != 'it' ) {

      echo "<style>#menu-item-9452 { display: none; } </style>";

  }

?>

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