How can I get Sprout Forms errors in current language using AJAX?
The AJAX POST url variable (postUrl) should be set from the form template with this Twig code:

&lt;script type="text/javascript"&gt;
    var postUrl;
    if ('{{ craft.i18n.getCurrentLocale() }}' == '{{ craft.i18n.getPrimarySiteLocale() }}') {
        postUrl = '/';
    } else {
        postUrl = '/{{ craft.i18n.getCurrentLocale() }}/';
    }
&lt;/script&gt;


Furthermore, an hidden field should be added in order to call the Sprout Forms' controller on submit:

&lt;input type="hidden" name="action" value="sproutForms/entries/saveEntry"&gt;


This submits the form in the current locale, and returns errors in this same locale.