Multi-lingual page structure was implemented according this tutorial: Building Sites in Multiple Languages

Here's my snippet of the main navigation:

<nav class="nav-lng">
    <a href="<?php echo View::url('/'); ?>" class="btn lng-fi spr"></a>
    <a href="<?php echo View::url('/en/home'); ?>" class="btn lng-en spr"></a>
</nav>

<nav class="nav-main">
<?php
    $nav = BlockType::getByHandle('autonav');
    if ($c->getCollectionAttributeValue('in_english'))
    {
        $nav->controller->displayPages = 'custom';
        $nav->controller->displayPagesCID = '123'; // additional lng stackparent's  ID here!
    }
    else
    {
        $nav->controller->displayPages = 'top';
    }
    $nav->controller->orderBy = 'display_asc';
    $nav->controller->displaySubPages = 'none';
    $nav->render('templates/select_path'); // my custom template's name
?>
</nav>

For hierarchically aware "current page trail" autonav I used a modified template, as community member 'ecomatt' had posted in his discussion Highlight Auto Nav current page and parent pages.

Here is the snippet, save it under /blocks/autonav/templates. I used the file name 'select_path'.

<?php 
   defined('C5_EXECUTE') or die(_("Access Denied."));
   $aBlocks = $controller->generateNav();
   $c = Page::getCurrentPage();
   $containsPages = false;

   $nh = Loader::helper('navigation');

   //this will create an array of parent cIDs 
   $inspectC=$c;
   $selectedPathCIDs=array( $inspectC->getCollectionID() );
   $parentCIDnotZero=true;   
   while($parentCIDnotZero){
      $cParentID=$inspectC->cParentID;
      if(!intval($cParentID)){
         $parentCIDnotZero=false;
      }else{
         $selectedPathCIDs[]=$cParentID;
         $inspectC=Page::getById($cParentID);
      }
   }    

   foreach($aBlocks as $ni) {
      $_c = $ni->getCollectionObject();
      if (!$_c->getCollectionAttributeValue('exclude_nav')) {
         if (!$containsPages) {
            // this is the first time we've entered the loop so we print out the UL tag
            echo("<ul class=\"nav\">");
         }

         $containsPages = true;

         $thisLevel = $ni->getLevel();
         if ($thisLevel > $lastLevel) {
            echo("<ul>");
         } else if ($thisLevel < $lastLevel) {
            for ($j = $thisLevel; $j < $lastLevel; $j++) {
               if ($lastLevel - $j > 1) {
                  echo("</li></ul>");
               } else {
                  echo("</li></ul></li>");
               }
            }
         } else if ($i > 0) {
            echo("</li>");
         }

         $pageLink = false;

         if ($_c->getCollectionAttributeValue('replace_link_with_first_in_nav')) {
            $subPage = $_c->getFirstChild();
            if ($subPage instanceof Page) {
               $pageLink = $nh->getLinkToCollection($subPage);
            }
         }

         if (!$pageLink) {
            $pageLink = $ni->getURL();
         }


         // MODIFIED!  this is the new function that allows nav-selected as well as nav-path-selected

         if ($ni->isActive($c) || ($c->getCollectionID() == $_c->getCollectionID()))
         {  
            echo('<li class="nav-selected nav-path-selected"><a class="nav-selected nav-path-selected" href="' . $pageLink . '">' . $ni->getName() . '</a>');
         }
         elseif ( in_array($_c->getCollectionID(),$selectedPathCIDs) && $_c->getCollectionID() != 1 )
         { 
            echo('<li class="nav-path-selected"><a class="nav-path-selected" href="' . $pageLink . '">' . $ni->getName() . '</a>');
         }
         else
         {
            echo('<li><a href="' . $pageLink . '">' . $ni->getName() . '</a>');
         }   

         $lastLevel = $thisLevel;
         $i++;


      }
   }

   $thisLevel = 0;
   if ($containsPages) {
      for ($i = $thisLevel; $i <= $lastLevel; $i++) {
         echo("</li></ul>");
      }
   }

?>

Now you can hook with CSS on the class '.nav-path-selected' on both the 'li' and 'a' elements.

And finally my page template 'left_sidebar.php' in my custom theme's folder:

<?php defined('C5_EXECUTE') or die(_("Access Denied.")); 
    $this->inc('elements/header.php');
?>

<nav class="nav-sub">
<?php
    $nh                     = Loader::helper('navigation');
    $trail                  = $nh->getTrailToCollection($c);
    //count how many levels down the hierarchy is the current page
    $levels                 = count($trail);
    $curID                  = $c->getCollectionID();
    $parentID               = $c->getCollectionParentID();

    // translated pages have a checkbox attribute set
    // if translated page, these reside in the own stack adding one more level to hierarchy
    $mod                    = ($c->getCollectionAttributeValue('in_english')) ? 1 : 0;

    // in this code there is only mainpage and subpage levels visible in their own navigations
    // you could add the grandParent by getting and saving it's ID on the var $grandParentID,
    // unquoting the two quotes below and changing the 'displaySubPageLevelsNum'

    $is1stLevel         = 1 + $mod;
    $is2ndLevel         = 2 + $mod;
    // $is3rdLevel      = 3 + $mod;

    if      ($levels == $is1stLevel)    { $disp = $curID; }
    else if ($levels == $is2ndLevel)    { $disp = $parentID; }
    // else if ($levels == $is3rdLevel) { $disp = $grandParentID; }

    $sn = BlockType::getByHandle('autonav');
    $sn->controller->displayPages = 'custom';
    $sn->controller->displayPagesCID = $disp;
    $sn->controller->displaySubPageLevelsNum = '0'; //change here if needed
    $sn->controller->orderBy = 'display_asc';
    $sn->render('templates/header_menu');
?>
</nav>
<section id="content" role="main">
<?php
    $b = new Area('Main');
    $b->display($c);
?>
</section>
<?php $this->inc('elements/footer.php'); ?>