// Here is my plugin's init function, where it adds the endpoint:

$langSwitchPermalink = "langswitch_lang";

function langswitch_init () {
   global $langSwitchPermalink;
   global $wp_rewrite;

   // this doesn't seem to work in global namespace
   add_rewrite_endpoint( $langSwitchPermalink, 
                         EP_PERMALINK +
                         EP_DATE + 
                         EP_YEAR +
                         EP_MONTH +
                         EP_DAY +
                         EP_ROOT +
                         EP_COMMENTS +
                         EP_SEARCH + 
                         EP_CATEGORIES +
                         EP_AUTHORS + 
                         EP_PAGES );
   $wp_rewrite->flush_rules();
}

// The actual permalink sections are added with filters, e.g.:

add_filter( 'the_permalink', 'langswitch_add_language_url' );
add_filter( 'post_link', 'langswitch_add_language_url' );

// Definition of that function (somewhat simplified -- the real version
// does a lot more checking

$langswitch_lang = "es";

function langswitch_add_language_url( $urlstr ) 
{
  global $langSwitchPermalink, $langswitch_lang;

  return langswitch_trailingslashit( $urlstr ) . $langSwitchPermalink . "/" .
      $langswitch_lang;
}
  
function langswitch_trailingslashit( $uri ){
  if( $uri{ strlen($uri) - 1 } != '/' ) {
    return $uri . '/';
  } else {
    return $uri;
  }
}


/////////////////////////////////
Here is a printout of the rewrite rules being generated, with the 
permalink set to /%category%/%postname%/ and a couple of pages present in the database.

Current blog URL is  http://poplarware.com/WPTest/
I also have a hack installed on that page that prints 
out which rule matched whenever you visit a link; the rewrite
rules are also printed out at the top of every post. Here they 
are currently:

robots.txt$
wp-atom.php$
wp-rdf.php$
wp-rss.php$
wp-rss2.php$
wp-feed.php$
wp-commentsrss2.php$
(another-page/about)/trackback/?$
(another-page/about)/feed/(feed|rdf|rss|rss2|atom)/?$
(another-page/about)/(feed|rdf|rss|rss2|atom)/?$
(another-page/about)/page/?([0-9]{1,})/?$
(another-page/about)/langswitch_lang(/(.*))?/?$
(another-page/about)(/[0-9]+)?/?$
another-page/about/attachment/([^/]+)/?$
another-page/about/attachment/([^/]+)/trackback/?$
another-page/about/attachment/([^/]+)/feed/(feed|rdf|rss|rss2|atom)/?$
another-page/about/attachment/([^/]+)/(feed|rdf|rss|rss2|atom)/?$
(another-page)/trackback/?$
(another-page)/feed/(feed|rdf|rss|rss2|atom)/?$
(another-page)/(feed|rdf|rss|rss2|atom)/?$
(another-page)/page/?([0-9]{1,})/?$
(another-page)/langswitch_lang(/(.*))?/?$
(another-page)(/[0-9]+)?/?$
another-page/attachment/([^/]+)/?$
another-page/attachment/([^/]+)/trackback/?$
another-page/attachment/([^/]+)/feed/(feed|rdf|rss|rss2|atom)/?$
another-page/attachment/([^/]+)/(feed|rdf|rss|rss2|atom)/?$
(test-static-post-page)/trackback/?$
(test-static-post-page)/feed/(feed|rdf|rss|rss2|atom)/?$
(test-static-post-page)/(feed|rdf|rss|rss2|atom)/?$
(test-static-post-page)/page/?([0-9]{1,})/?$
(test-static-post-page)/langswitch_lang(/(.*))?/?$
(test-static-post-page)(/[0-9]+)?/?$
test-static-post-page/attachment/([^/]+)/?$
test-static-post-page/attachment/([^/]+)/trackback/?$
test-static-post-page/attachment/([^/]+)/feed/(feed|rdf|rss|rss2|atom)/?$
test-static-post-page/attachment/([^/]+)/(feed|rdf|rss|rss2|atom)/?$
feed/(feed|rdf|rss|rss2|atom)/?$
(feed|rdf|rss|rss2|atom)/?$
page/?([0-9]{1,})/?$
comments/feed/(feed|rdf|rss|rss2|atom)/?$
comments/(feed|rdf|rss|rss2|atom)/?$
comments/page/?([0-9]{1,})/?$
search/(.+)/feed/(feed|rdf|rss|rss2|atom)/?$
search/(.+)/(feed|rdf|rss|rss2|atom)/?$
search/(.+)/page/?([0-9]{1,})/?$
search/(.+)/langswitch_lang(/(.*))?/?$
search/(.+)/?$
category/(.+?)/feed/(feed|rdf|rss|rss2|atom)/?$
category/(.+?)/(feed|rdf|rss|rss2|atom)/?$
category/(.+?)/page/?([0-9]{1,})/?$
category/(.+?)/langswitch_lang(/(.*))?/?$
category/(.+?)/?$
author/([^/]+)/feed/(feed|rdf|rss|rss2|atom)/?$
author/([^/]+)/(feed|rdf|rss|rss2|atom)/?$
author/([^/]+)/page/?([0-9]{1,})/?$
author/([^/]+)/langswitch_lang(/(.*))?/?$
author/([^/]+)/?$
([0-9]{4})/([0-9]{1,2})/([0-9]{1,2})/feed/(feed|rdf|rss|rss2|atom)/?$
([0-9]{4})/([0-9]{1,2})/([0-9]{1,2})/(feed|rdf|rss|rss2|atom)/?$
([0-9]{4})/([0-9]{1,2})/([0-9]{1,2})/page/?([0-9]{1,})/?$
([0-9]{4})/([0-9]{1,2})/([0-9]{1,2})/langswitch_lang(/(.*))?/?$
([0-9]{4})/([0-9]{1,2})/([0-9]{1,2})/?$
([0-9]{4})/([0-9]{1,2})/feed/(feed|rdf|rss|rss2|atom)/?$
([0-9]{4})/([0-9]{1,2})/(feed|rdf|rss|rss2|atom)/?$
([0-9]{4})/([0-9]{1,2})/page/?([0-9]{1,})/?$
([0-9]{4})/([0-9]{1,2})/langswitch_lang(/(.*))?/?$
([0-9]{4})/([0-9]{1,2})/?$
([0-9]{4})/feed/(feed|rdf|rss|rss2|atom)/?$
([0-9]{4})/(feed|rdf|rss|rss2|atom)/?$
([0-9]{4})/page/?([0-9]{1,})/?$
([0-9]{4})/langswitch_lang(/(.*))?/?$
([0-9]{4})/?$
(.+?)/([^/]+)/trackback/?$
(.+?)/([^/]+)/feed/(feed|rdf|rss|rss2|atom)/?$
(.+?)/([^/]+)/(feed|rdf|rss|rss2|atom)/?$
(.+?)/([^/]+)/page/?([0-9]{1,})/?$
(.+?)/([^/]+)/langswitch_lang(/(.*))?/?$
(.+?)/([^/]+)(/[0-9]+)?/?$
.+?/[^/]+/([^/]+)/?$
.+?/[^/]+/([^/]+)/trackback/?$
.+?/[^/]+/([^/]+)/feed/(feed|rdf|rss|rss2|atom)/?$
.+?/[^/]+/([^/]+)/(feed|rdf|rss|rss2|atom)/?$
.+?/[^/]+/attachment/([^/]+)/?$
.+?/[^/]+/attachment/([^/]+)/trackback/?$
.+?/[^/]+/attachment/([^/]+)/feed/(feed|rdf|rss|rss2|atom)/?$
.+?/[^/]+/attachment/([^/]+)/(feed|rdf|rss|rss2|atom)/?$
(.+?)/feed/(feed|rdf|rss|rss2|atom)/?$
(.+?)/(feed|rdf|rss|rss2|atom)/?$
(.+?)/page/?([0-9]{1,})/?$
(.+?)/langswitch_lang(/(.*))?/?$
(.+?)/?$