Add support for HTTP Accept-Language

Allows site to auto-detect language based on HTTP Accept-Language header.

Uses a lightly modified version of the parser from
http://www.thefutureoftheweb.com/blog/use-accept-language-header.

Also cleans up unnecessary direct use of $_COOKIE and $_GET, which
are both covered by $_REQUEST, and uses a single language fallback
instead of a default + fallback.

index.php

@@ -3,17 +3,47 @@
  * Multilingual support
  */
 global $lang;
-/* Default to English */
-$lang = 'en';
-/* Check if the user has set a language preference before (cookies) */
-if (!empty($_COOKIE['lang']))
-  $lang = $_COOKIE['lang'];
-/* The GET language parameter should override any stored setting */
-if (!empty($_GET['lang']))
-  $lang = $_GET['lang'];
-/* Make sure that the language is known, otherwise fall back to English */
-if (!preg_match("/^([a-z][a-z]|[a-z][a-z]_[a-z][a-z][a-z]?)$/", $lang))
-	$lang = "en";
+
+$available_languages = array(
+  'en' => true,
+  'de' => true,
+  'fr' => true,
+  'it' => true,
+  'ru' => true
+);
+
+function get_preferred_languages() {
+  $lang_parse = array();
+  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])) {
+    $langs = array_combine($lang_parse[1], $lang_parse[4]);
+    foreach ($langs as $candidate => $quality) {
+      if ($quality === '') {
+        $langs[$candidate] = 1;
+      }
+    }
+    arsort($langs, SORT_NUMERIC);
+    return array_keys($langs);
+  }
+
+  return array();
+}
+
+if (!empty($_REQUEST['lang'])) {
+  $lang = $_REQUEST['lang'];
+} elseif (isset($_SERVER['HTTP_ACCEPT_LANGUAGE'])) {
+  foreach (get_preferred_languages() as $candidate) {
+    $candidate_major = current(explode('-', $candidate, 1));
+    if (isset($available_languages[$candidate_major])) {
+      $lang = $candidate_major;
+      break;
+    }
+  }
+}
+
+if (!array_key_exists($lang, $available_languages))
+  $lang = 'en';
 
 /* We have to clean the mess introduced by double cookie at the wrong level */
 if (empty($_COOKIE['clear_lang'])) {