P.S. If you want a fix before the next release, the following fix should take care of all AJAX-based unicode issues.  I have tested this with Russian and Hindi so far, and everything seems to be in order. Please report any bugs you may encounter.

1. Open /core/class_global.php, insert the following before function array_stripslashes():

function unicode_urldecode($str)
{
    $str = preg_replace("%u([0-9a-f]{3,4})/i","", urldecode($str) );
    return html_entity_decode($str, null, 'UTF-8');
}

2. Open /core/ajax/functions.php, insert after the script header comments the following:

if (!empty($_POST))
{
    foreach ($_POST as $key=>$val)
    {
        if (strpos($val, "%") !== false)
            $_POST[$key] = Utility::unicode_urldecode($val);
    }
}