add `navigator-language` template variable

src/ui/pages/help.html

@@ -838,6 +838,7 @@
 					`{page-title}[10ch]` to limit the title to 10 characters).
 				</p>
 				<ul>
+					<li><code>{navigator-language}</code>: the language of the browser</li>
 					<li><code>{page-title}</code>: the title of the page</li>
 					<li><code>{page-heading}</code>: the content of the H1 tag in the page</li>
 					<li><code>{page-language}</code>: the language of the page</li>