Now I get it clearly! I've been working for a few hours on a possible solution for your problem, and I think I'm getting closer :) I have to go out right now, but I'll keep working on this as soon as I can, I promise!
WordPress gives me big headaches, but I really like learning it. After trying hard, I finally got how to register a menu so it works fine on the CMS, using the function refence wp_page_menu(). Using this, you can register your second menu so you'll be able to use it inside your theme. Here's a sample of what you could do, open your functions.php file and add a secondary menu right after the primary:
register_nav_menus( array(
'primary' => __( 'Primary Navigation', 'twentyten' ),
'english' => __( 'English Navigation', 'twentyten' ),
) );
This is what I acchieved so far. I know it's not much but it took me a while to define it correctly for my theme. In the Admin panel, go to Appereance > Menus. If you can see a second slot in "Theme Location" saying "English Navigation", it means the registration worked out :)
Now we just need to find the place to put this code:
<?php wp_nav_menu( array( 'container_class' => 'menu-header-english', 'theme_location' => 'english' ) ); ?>
and make the pages recognize when to use the English NavBar. I hope I can get the solution soon!