Link Japanese translation

See triple-underscore/triple-underscore.github.io#1 (comment).

Author: annevk

index.bs

@@ -15,6 +15,7 @@ Abstract: as a guide to implementors of already-published specifications,
 Abstract: and that newly published specifications reference this
 Abstract: document to ensure conforming implementations of interfaces
 Abstract: are interoperable.
+Translation: ja https://triple-underscore.github.io/WebIDL-ja.html
 Complain About: accidental-2119 no
 </pre>