Oh, you're using JoomFish? In this case made such modification:
Open file /components/com_jcomments/jcomments.class.php and replace:
return JURI::root(true) . '/index.php?option=com_jcomments&task=captcha&tmpl=component&ac=' . $random;
with
$lang = ($mainframe->getCfg('multilingual_support') == 1) ? ('&lang='.$iso_client_lang) : '';
return JURI::root(true) . '/index.php?option=com_jcomments&task=captcha&tmpl=component&ac=' . $random. $lang;