Add one or more classes to the WordPress comment form's 'Submit' button

modify-comments.php

<?php

/**
 * Add one or more classes to the WordPress comment form's 'Submit' button
 * NB: This only works as of WordPress 4.1
 * @author Alain Schlesser (alain.schlesser@gmail.com)
 * @link http://www.brightnucleus.com/add-class-wordpress-comment-submit-button/
 *
 * @param  array  $arg  contains all the default values used by the comment form
 * @return array        modified array that goes back to the comment renderer
 *
 * @see http://codex.wordpress.org/Function_Reference/comment_form
 * @see http://developer.wordpress.org/reference/hooks/comment_form_defaults/
 */
function as_adapt_comment_form( $arg ) {
    // $arg contains all the comment form defaults
    // simply redefine one of the existing array keys to change the comment form
    // see http://codex.wordpress.org/Function_Reference/comment_form for a list
    // of array keys

    // add Foundation classes to the button class
    $arg['class_submit'] = 'button radius';

    // return the modified array
    return $arg;
}

// run the comment form defaults through the newly defined filter
add_filter( 'comment_form_defaults', 'as_adapt_comment_form' );

?>

See article at the Bright Nucleus Homepage:

• Add a Class to the WordPress Comment Submit Button