WHMCS comes with a lot of available languages for its interface, few of them also quite exotic.
So many languages may not to be useful, especially if your system is specially addressed to a specific country.

The quick and easy way to reduce the number of languages requires simply to delete the files/languages you don't need from the lang folder.
This way, the delete language(s) will be removed from the menu.

Beware: when WHMCS will be updated, the deleted languages files will be restored; so these language files will be needed to be deted again.

Another (and more elegant) way is to use an action hook:

function locales_hook($vars) {
     $mylocal = $vars['locales'];
     foreach ($mylocal as $key => $value) { 
        if ($value["language"] != "english" && $value["language"] != "italian" && $value["language"] != "deutsch") {
            unset($mylocal[$key]); 
        }
     }
    return array("locales" => $mylocal);
}
add_hook("ClientAreaPage", 1, "locales_hook");

This hook will show only the specified languages (in the sample: english, italian, deutsch)