How to change ajax-charset?

You could use:

contentType:"application/x-javascript; charset:ISO-8859-1"

By giving the content type explicitly during ajax call as below may allow you to override the default content type.

$.ajax({
        data: parameters,
        type: "POST",
        url: ajax_url,
        timeout: 20000,
        contentType: "application/x-www-form-urlencoded;charset=ISO-8859-15",
        dataType: 'json',
        success: callback
});

You would also have to specify the charset on the server.

Ex: for php

<?php header('Content-Type: text/html; charset=ISO-8859-15'); ?>

I hope this may help you.

It seems the charset cannot be changed anymore – $.ajax docs states:

The W3C XMLHttpRequest specification dictates that the charset is always UTF-8; specifying another charset will not force the browser to change the encoding.