If page is empty than it works like a trigger to open sublevel pages with content

[Everyone]

I had a simmilar need, so I came up with this. It uses i18n plugin's navigation API functions.

PHP Code:

function custom_tree_nav( $current_slug = null, $entries = false )
{
    $current_slug = $current_slug ? $current_slug : get_page_slug( false );

    if( !$entries )
        $entries = return_i18n_menu_data( $current_slug, 0, 99, I18N_SHOW_MENU );

    echo '<ul>';
    foreach( $entries as $entry )
    {
        # 'current' menu entry class tag.
        $current = $entry['current'] ? ' current' : '';

        # Get menu name, or page name if menu name empty.
        $menu = $entry['menu'] ? $entry['menu'] : $entry['title'];

        # This may need to be adjusted according to installation specifics
        $slug = '/'.$entry['url'];

        $url = $entry['url'];

        if( !empty( $entry['children'] ) )
        {
            # Might want to throw an 'a' instead of 'span'.
            echo '<li><span class="'.$url.$current.'">'.$menu.'</span>';
            custom_tree_nav( $current_slug, $entry['children'] );
            echo '</li>'."\n";
        }
        
        else
            echo '<li><a class="'.$url.$current.'" href="'.$slug.'" target="_self">'.$menu.'</a></li>'."\n";
    }
    echo '</ul>'."\n";
} 


Put this function in your theme's functions.php file, then call it from your template file like this: <?php custom_tree_nav(); ?>

It will create the raw HTML menu code. Be aware that no checks are made whether a page is empty or not, rather a page is checked for existence of children. Also, no attempt was made to recreate the url structure, as GS will (with proper configuration) accept anything after the last slash as a page slug, and return the correct page.

Any CSS styling + JS behavior is up to you.