<?php
// Import PHPMailer classes into the global namespace
// These must be at the top of your script, not inside a function
use PHPMailerPHPMailerPHPMailer;
use PHPMailerPHPMailerException;

require 'vendor/autoload.php';
if(isset($_POST['enviar'])) {
    if ($_POST['no-spam'] != ''){
        exit();
    }else {
        if (isset($_POST['name'], $_POST['email'], $_POST['comments'])){
            $mail = new PHPMailer(true);
            try {
                //Server settings
                $mail->SMTPDebug = 3;
                $mail->isSMTP();
                $mail->Host = '';
                $mail->SMTPAuth = true;
                $mail->Username = '';
                $mail->Password = '';
                $mail->SMTPSecure = 'ssl';
                $mail->Port = 465;
                $mail->SMTPOptions = array('ssl' => array( 'verify_peer' => false, 'verify_peer_name' => false, 'allow_self_signed' => true ));
                //Recipients
                $mail->setFrom('', 'Formulario de contacto');
                $mail->addAddress('');

                //Content
                $mail->isHTML(false);
                $mail->Subject = 'Cliente: '.$_POST['email'];
                $mail->Body    = $_POST['comments'];
                $mail->AltBody = $_POST['comments'];;

                $mail->send();
                header('Location: ../index_en.html');

        } catch (Exception $e) {
            echo 'Message could not be sent. Mailer Error: ', $mail->ErrorInfo;
        }
    }

}

<?php
// según yo, el autoload debe ir primero para que funcione "use PHPMailer..." pero si te funciona...
require 'vendor/autoload.php';

use PHPMailerPHPMailerException;
use PHPMailerPHPMailerPHPMailer;

// esto lo puedes dar por subentendido
// if(isset($_POST['enviar']))

function sendEmail()
{
    if (!isset($_POST['no-spam']) || $_POST['no-spam'] != '') {
        return ['status' => 'error', 'msg' => 'parece que es spam'];

    }
    if (!(isset($_POST['name'], $_POST['email'], $_POST['comments']))) {
        return ['status' => 'error', 'msg' => 'complete todos los campos'];
    }

    $mail = new PHPMailer(true);
    try {
        //Server settings
        $mail->SMTPDebug = 3;
        $mail->isSMTP();
        $mail->Host        = '';
        $mail->SMTPAuth    = true;
        $mail->Username    = '';
        $mail->Password    = '';
        $mail->SMTPSecure  = 'ssl';
        $mail->Port        = 465;
        $mail->SMTPOptions = ['ssl' => ['verify_peer' => false, 'verify_peer_name' => false, 'allow_self_signed' => true]];
        //Recipients
        $mail->setFrom('', 'Formulario de contacto');
        $mail->addAddress('');

        //Content
        $mail->isHTML(false);
        $mail->Subject = 'Cliente: ' . $_POST['email'];
        $mail->Body    = $_POST['comments'];
        $mail->AltBody = $_POST['comments'];

        $mail->send();
        return ['status' => 'OK', 'msg' => 'mail enviado'];

    } catch (Exception $e) {
        return ['status' => 'error', 'msg' => '$mail->ErrorInfo'];
    }

}

echo json_encode(sendEmail());