Add real-world example of using custom properties with :lang.

Author: tabatkins

css-variables/Overview.bs

@@ -240,6 +240,24 @@ Custom Property Value Syntax</h3>
 		</pre>
 	</div>
 
+	<div class='example'>
+		A real-world example of <a>custom property</a> usage
+		is easily separating out strings from where they're used,
+		to aid in maintenance of internationalization:
+
+		<pre class='lang-css'>
+		:root,
+		:root:lang(en) {--external-link: "external link";}
+		:root:lang(de) {--external-link: "externer Link";}
+
+		a[href^="http"]::after {content: " (" var(--external-link) ")"}
+		</pre>
+
+		The variable declarations can even be kept in a separate file,
+		to make maintaining the translations simpler.
+	</div>
+
+
 
 <h3 id='cycles'>
 Resolving Dependency Cycles</h3>