LangRouter.php

<?php
/* LangRouter
 * ==========
 *
 * This plugin is meant to be used with Babel extra for MODX Revolution. It 
 * takes care of switching contexts, which hold translations, depending on URL 
 * requested by client. LangRouter works with so called subfolder based setup, 
 * in which many languages are served under a single domain but are 
 * differentiated by a virtual subfolder indicating the language, eg. 
 * mydomain.com/pl/.
 *
 * The routing work as follows:
 * - if URI contains cultureKey, which is defined in Babel configuration, then 
 *   the matching context is served
 * - if URI doesn't contain cultureKey (or one not defined in Babel 
 *   configuration) AND at least one of the client's accepted languages is 
 *   defined in Babel configuration, then the matching context is served
 * - otherwise the default context is served
 *
 * LangRouter works out-of-the-box and doesn't require any changes to URL 
 * rewrite rules in the webserver configuration. All routing is handled 
 * internally by MODX. This greatly simplifies the setup and provides 
 * portability. LangRouter was tested with Apache and Lighttpd.
 *
 * Setup:
 * 1. Prepare your contexts as you normally would for Babel.
 * 2. For each context set `base_url` to `/`.
 * 3. For each context set `site_url` to 
 *    `{server_protocol}://{http_host}{base_url}{cultureKey}/`
 * 4. Add new system setting `babel.contextDefault` and set it to the default 
 *    context, which should be served when no language is specified in 
 *    request, eg. `pl`.
 * 5. Include static files from the assets folder with 
 *    `[[++assets_url]]path/to/static_file`.
 * 6. In template header use `<base href="[[++site_url]]" />`.
 * 7. Use default URL generation scheme in MODX (ie. relative).
 *
 * This code is shared AS IS. Use at your own risk.
 */

if($modx->context->get('key') != "mgr") {

  /*
   * Debugs request handling
   */
  function logRequest($message = 'Request')
  {
    global $modx;
    $modx->log(modX::LOG_LEVEL_ERROR, $message . ':'
      . "\n REQUEST_URI:   " . $_SERVER['REQUEST_URI']
      . "\n REDIRECT_URI:  " . $_SERVER['REDIRECT_URI']
      . "\n QUERY_STRING:  " . $_SERVER['QUERY_STRING']
      . "\n q:             " . $_REQUEST['q']
      . "\n Context:       " . $modx->context->get('key')
      . "\n Site start:    " . $modx->context->getOption('site_start')
    );
  }

  /*
   * Dumps variables to MODX log
   */
  function dump($var) {
    ob_start();
    var_dump($var);
    return ob_get_clean();
  }

  /*
   * Detects client language preferences and returns associative array sorted 
   * by importance (q factor)
   */
  function clientLangDetect()
  {
    $langs = array();

    if (isset($_SERVER['HTTP_ACCEPT_LANGUAGE'])) {
      # break up string into pieces (languages and q factors)
      preg_match_all('/([a-z]{1,8}(-[a-z]{1,8})?)\s*(;\s*q\s*=\s*(1|0\.[0-9]+))?/i', $_SERVER['HTTP_ACCEPT_LANGUAGE'], $lang_parse);

      if (count($lang_parse[1])) {
        # create a list like "en" => 0.8
        $langs = array_combine($lang_parse[1], $lang_parse[4]);

        # set default to 1 for any without q factor
        foreach ($langs as $lang => $val) {
          if ($val === '') $langs[$lang] = 1;
        }

        # sort list based on value
        arsort($langs, SORT_NUMERIC);
        return $langs;
      }
    }
  }

  #logRequest('Unhandled request');

  # Get contexts and their cultureKeys
  $babelContexts = explode(',', $modx->getOption('babel.contextKeys'));
  $languages = array();
  foreach ($babelContexts as $context) {
    $ctx = $modx->getContext($context);
    $languages[$ctx->config['cultureKey']] = trim($context);
  }
  #$modx->log(modX::LOG_LEVEL_ERROR, dump($languages));

  # Determine language from request
  $reqCultureKeyIdx = strpos($_REQUEST['q'], '/');
  $reqCultureKey = substr($_REQUEST['q'], 0, $reqCultureKeyIdx);

  # Serve the proper context and language
  if(array_key_exists(
    strtolower($reqCultureKey), array_change_key_case($languages))) {
      $modx->switchContext($reqCultureKey);
      # Remove cultureKey from request
      $_REQUEST['q'] = substr($_REQUEST['q'], $reqCultureKeyIdx+1);
      #logRequest('Culture key found in URI');
    } else {
      $clientCultureKey = array_flip(
        array_intersect_key(clientLangDetect(), $languages));
      if($clientCultureKey) {
        $contextDefault = current($clientCultureKey);
      } else {
        $contextDefault = trim($modx->getOption('babel.contextDefault'));
      }
      #$modx->log(modX::LOG_LEVEL_ERROR, dump($contextDefault));
      $modx->switchContext($contextDefault);
      #logRequest('Culture key not found in URI');
      $modx->sendRedirect($modx->context->getOption('site_url'));
    }

  # Serve site_start when no resource is requested
  if(empty($_REQUEST['q'])) {
    #$modx->log(modX::LOG_LEVEL_ERROR, 'Query is empty');
    $modx->sendForward($modx->context->getOption('site_start'));
  }

}

Just to save some time for others there are no changes in this fork.