In some case, clicking on a link generated by mainNavigation component may lead into a 404 error.
That happens when no culture is set yet.
To correct this, replace your /plugins/sfSimpleCMSPlugin/modules/sfSimpleCMS/templates/_mainNavigation.php content with this one below :
<ul class="site_navigation">
<?php foreach ($level1_nodes as $node): ?>
<li <?php if ($node->getSlug() == $page->getSlug()): ?>class="current"<?php endif; ?>>
<?php echo link_to(
$node->__toString($culture),
sfSimpleCMSTools::urlForPage($node->getSlug(), null, $culture),
array('class' => 'cms_page_navigation')
) ?>
</li>
<?php endforeach; ?>
</ul>
Or download filed attached and replace your /plugins/sfSimpleCMSPlugin/modules/sfSimpleCMS/templates/_mainNavigation.php
