Added hint for i18n-duplication

Author: aschempp

docs/changelanguage/_index.en.md

@@ -13,6 +13,10 @@ number of page trees in different languages on either the same or different
 domains. *ChangeLanguage* simply adds the functionality to link pages
 together and present the website visitor with a list of available languages.
 
+If your workflow consists of creating a page in one language and then
+duplicating it for translation, take a look at the [i18n-duplication][2]
+extension.
 
 
 [1]: https://contao.org
+[2]: https://extensions.contao.org/?p=christianbargon/contao-i18n-duplication