english for admin, chinese for users in drupal 6

Backport from D7 patch to D6
Edit: updated for #53

/**
 * Identify language from the Accept-language HTTP header we got.
 */
function language_from_browser() {
  // Get enabled language and create an array of valid language objects.
  $languages = language_list('enabled');
  $languages = $languages['1'];
  
  if (!isset($_SERVER['HTTP_ACCEPT_LANGUAGE'])) {
    return;
  }

  // RFC 2616 (section 14.4) defines the Accept-Language header as followed:
  //   Accept-Language = "Accept-Language" ":"
  //                  1#( language-range [ ";" "q" "=" qvalue ] )
  //   language-range  = ( ( 1*8ALPHA *( "-" 1*8ALPHA ) ) | "*" )
  // Samples: "hu, en-us;q=0.66, en;q=0.33", "hu,en-us;q=0.5"
  $browser_langs = array();
  if (preg_match_all('@([a-zA-Z-]+|\*)(?:;q=([0-9.]+))?(?:$|\s*,\s*)@', $_SERVER['HTTP_ACCEPT_LANGUAGE'], $matches, PREG_SET_ORDER)) {
    foreach ($matches as $match) {
      // We can safely use strtolower() here, tags are ASCII.
      // RFC2616 mandates that the decimal part is no more than three digits,
      // so we multiply the qvalue by 1000 to avoid floating point comparisons.
      $langcode = strtolower($match[1]);
      $qvalue = isset($match[2]) ? (float) $match[2] : 1;
      $browser_langs[$langcode] = (int) ($qvalue * 1000);
    }
  }

  // Some browsers (especially some versions of Internet Explorer) sometimes
  // send a specific language tag (fr-CA) without the corresponding generic
  // tag (fr). In that case, we assume that the lowest value of the specific
  // tags is the value of the generic language.
  arsort($browser_langs);
  foreach ($browser_langs as $langcode => $qvalue) {
    $generic_tag = strtok($langcode, '-');
    if (!isset($browser_langs[$generic_tag])) {
      $browser_langs[$generic_tag] = $qvalue;
    }
  }

  // Find the enabled language with the greatest qvalue, following the rules
  // of RFC 2616 (section 14.4). If several languages have the same qvalue,
  // prefer the one with the greatest weight.
  $best_match = NULL;
  $max_qvalue = 0;
  foreach ($languages as $langcode => $language) {
    // Language tags are case insensitive (RFC2616, sec 3.10).
    $langcode = strtolower($langcode);

    // If nothing matches below, the default qvalue is the one of the wildcard
    // language, if set, or is 0 (which will never match).
    $qvalue = isset($browser_langs['*']) ? $browser_langs['*'] : 0;

    // Find the longest possible prefix of the browser-supplied language
    // ('the language-range') that matches this site language ('the language tag').
    $prefix = $langcode;
    do {
      if (isset($browser_langs[$prefix])) {
        $qvalue = $browser_langs[$prefix];
        break;
      }
    }
    while ($prefix = substr($prefix, 0, strrpos($prefix, '-')));

    // Find the best match.
    if ($qvalue > $max_qvalue) {
      $best_match = $language->language;
      $max_qvalue = $qvalue;
    }
  }
  
  return $languages[$best_match];
}