Override to add privacy policy checkbox to com_contact

default_form.php

<?php
/**
 * @author     Carlos Cámara
 * @url  https://www.hepta.es
 *
 * @copyright   Copyright (C) 2005 - 2017 Open Source Matters, Inc. All rights reserved.
 * @license     GNU General Public License version 2 or later; see LICENSE.txt
 */

defined('_JEXEC') or die;

JHtml::_('behavior.keepalive');
JHtml::_('behavior.formvalidator');

?>
<div class="contact-form">
	<form id="contact-form" action="<?php echo JRoute::_('index.php'); ?>" method="post" class="form-validate form-horizontal well">
		<?php foreach ($this->form->getFieldsets() as $fieldset) : ?>
			<?php if ($fieldset->name === 'captcha' && !$this->captchaEnabled) : ?>
				<?php continue; ?>
			<?php endif; ?>
			<?php $fields = $this->form->getFieldset($fieldset->name); ?>
			<?php if (count($fields)) : ?>
				<fieldset>
					<?php if (isset($fieldset->label) && ($legend = trim(JText::_($fieldset->label))) !== '') : ?>
						<legend><?php echo $legend; ?></legend>
					<?php endif; ?>
					<?php foreach ($fields as $field) : ?>
						<?php echo $field->renderField(); ?>
					<?php endforeach; ?>
                  <div class="control-group">
                    <div class="control-label">
                    <?php if ($this->contact->params->get('linke')) : ?>
                    	<label id="jform_contact_email_accept-lbl" for="jform_contact_email_accept" class="hasPopover required" title="" data-content="Debe aceptar las condiciones de uso para poder continuar" data-original-title="Aceptar condiciones de uso" aria-invalid="false">
                        Acepto las <a href="<?= $this->contact->params->get('linke');?>" target="_blank">condiciones de uso</a></label>
                    <?php else :?>
                    	<label id="jform_contact_email_accept-lbl" for="jform_contact_email_accept" class="hasPopover required" title="" data-content="Debe aceptar las condiciones de uso para poder continuar" data-original-title="Aceptar condiciones de uso" aria-invalid="false">
                        Acepto las condiciones de uso</label>
                    <?php endif; ?>
                    <span class="star">&nbsp;*</span>
                    </div>
                    <div class="controls"><input name="jform[contact_email_accept]" id="jform_contact_email_accept" class="required" value="1" type="checkbox"></div>
                   </div>                  	
				</fieldset>
			<?php endif; ?>
		<?php endforeach; ?>
		<div class="control-group">
			<div class="controls">
				<button class="btn btn-primary validate" type="submit"><?php echo JText::_('COM_CONTACT_CONTACT_SEND'); ?></button>
				<input type="hidden" name="option" value="com_contact" />
				<input type="hidden" name="task" value="contact.submit" />
				<input type="hidden" name="return" value="<?php echo $this->return_page; ?>" />
				<input type="hidden" name="id" value="<?php echo $this->contact->slug; ?>" />
				<?php echo JHtml::_('form.token'); ?>
			</div>
		</div>
	</form>
</div>