Fix a bug when there no language sent (many OPDS client do that).

base.php

@@ -48,7 +48,11 @@ function localize($phrase) {
     static $translations = NULL;
     /* If no instance of $translations has occured load the language file */
     if (is_null($translations)) {
-        $lang = substr($_SERVER['HTTP_ACCEPT_LANGUAGE'], 0, 2);
+        $lang = "en";
+        if (isset($_SERVER['HTTP_ACCEPT_LANGUAGE']))
+        {
+            $lang = substr($_SERVER['HTTP_ACCEPT_LANGUAGE'], 0, 2);
+        }
         $lang_file_en = NULL;
         $lang_file = 'lang/Localization_' . $lang . '.json';
         if (!file_exists($lang_file)) {