Add a link to 'polyglot document'

Fix #540.

tutorials/language-decl/index.en.html

@@ -82,7 +82,7 @@ <h2 id="quicksummary">In a nutshell</h2>
 <div class="example">
 <p><code translate="no">&lt;html lang=&quot;en&quot;&gt;</code></p>
 </div>
-<p>If this is XHTML 1.x or an HTML5 polyglot document served as XML, you should also use the <code class="kw" translate="no">xml:lang</code> attribute (with the same value). If your page is only served as XML, just use the <code class="kw" translate="no">xml:lang</code> attribute.</p>
+<p>If this is XHTML 1.x or an HTML5 <a href="https://www.w3.org/TR/html-polyglot/#dfn-polyglot-markup">polyglot document</a> served as XML, you should also use the <code class="kw" translate="no">xml:lang</code> attribute (with the same value). If your page is only served as XML, just use the <code class="kw" translate="no">xml:lang</code> attribute.</p>
 <p>Do not use the <code class="kw" translate="no">meta</code> element with <code class="kw" translate="no">http-equiv</code> set to <code class="kw" translate="no">Content-Language</code>.</p>
 <p>Choose tags from the IANA Subtag Registry, and follow the syntax rules in BCP 47 when using more than just a language subtag. Keep your language tags as short as possible.</p>
 <p>Use the <code class="kw" translate="no">:lang</code> property in CSS to apply styles dependent on the language of your content. </p>