Is it possible to use multiple captcha based on language selection?
Well, you should do this in template.php. First check the language then add the captcha based on the language. As an example to do this in webforms:

function THEME_form_alter(&amp;$form, $form_state, $form_id) {
    global $language;
    if($language-&gt;name == "ENGLISH")
    { 
        if (strstr($form_id, 'webform_client_form')) {
            $form['my_captcha_element'] = array(
                '#type' =&gt; 'captcha',
                '#captcha_type' =&gt; 'image_captcha/Image',
            );
        }
     }
}


You can check the language and add the mollom programmatically as well. Follow this link to learn how to add mollom by code.