[css-variables-1] change to visually distinct language, fix w3c#7131

Author: svgeesus

css-variables-1/Overview.bs

@@ -308,7 +308,7 @@ Defining Custom Properties: the '--*' family of properties</h2>
 		<pre class='lang-css'>
 		:root,
 		:root:lang(en) {--external-link: "external link";}
-		:root:lang(de) {--external-link: "externer Link";}
+		:root:lang(el) {--external-link: "εξωτερικός σύνδεσμος";}
 
 		a[href^="http"]::after {content: " (" var(--external-link) ")"}
 		</pre>