Change array

https://github.com/PrivateBin/PrivateBin/issues/41

Not tested locally, let's say what Travis says... :smile:

lib/i18n.php

@@ -141,7 +141,7 @@ class i18n
         // check if the lang cookie was set and that language exists
         if (array_key_exists('lang', $_COOKIE) && in_array($_COOKIE['lang'], $availableLanguages))
         {
-            $match = $_COOKIE['lang'];
+            $match = $availableLanguages[$_COOKIE['lang']];
         }
         // find a translation file matching the browsers language preferences
         else