Revert "Enable Polish translation again", for now (#989)

This reverts commit 70bd5e0aa9.

While the docs do build, I belive there's at least a few specific action
points we should tackle before we open them up to the world.

include/languages.inc

@@ -30,6 +30,7 @@ $LANGUAGES = [
  - http://docs.php.net/
 */
 $INACTIVE_ONLINE_LANGUAGES = [
+    'pl' => 'Polish',
     'ro' => 'Romanian',
     'uk' => 'Ukrainian',
 ];