Specifying the content type on the AJAX-call solved my problems on a Norwegian site.
$.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.
<?php header('Content-Type: text/html; charset=ISO-8859-15'); ?>