[1.5.0] Add rudimentary I18N and L10N support based off MediaWiki

- Also: allow 'x' subtag in language codes

git-svn-id: http://htmlpurifier.org/svnroot/htmlpurifier/trunk@701 48356398-32a2-884e-a903-53898d9a118a

library/HTMLPurifier/Language/messages/en-x-test.php

@@ -0,0 +1,11 @@
+<?php
+
+// private language message file for unit testing purposes
+
+$fallback = 'en';
+
+$messages = array(
+    'htmlpurifier' => 'HTML Purifier X'
+);
+
+?>