The 'register_nav_menus' was correct. So you don't need to change it. The correct version for wp_nav_menu is then:
wp_nav_menu( array('theme_location' => 'main-menu',
'container_class' => 'nav-collapse',
'menu_class' => 'nav',
'fallback_cb' => '',
'walker' => new Bootstrapwp_Walker_Nav_Menu()
) );Thus you should have two theme locations:
Main menu English
Main menu Español
That's almost what you did but I believe you misunderstood the usage of the function __().
In addition, it seems that the "Theme locations" control in Appearance->Menus is not working
Are you using the latest version Polylang?
