Posting to plugin's action controller using Ajax + JSON
In your newsletter form, you should have:

&lt;input type="hidden" name="action" value="mailchimp-subscribe/audience/subscribe"&gt;
&lt;input type="hidden" name="language" value="{{ craft.app.locale.id }}"&gt;


Then, you should POST to '/' + userLocale (where userLocale = $('[name="language"]').val()).

I am using jQuery, and this is working fine:

$.post({
    url: '/' + userLocale,
    dataType: 'json',
    data: $('#form').serialize(),
    success: function(response) {
        console.log(response);
    },
    error: function(response) {
        console.log(response);
    }
});