Deployed during 2015-04-27 evening SWAT.
The link Zona de pruebas is well present, with <user page>/zona de pruebas as target.
@PeterBowman If you wish to customize the link, you can edit on wiki [[MediaWiki:sandboxlink-subpage-name]].
There is also the possibility to add a block of text before the edit area or to preload some text in the editor, when a new page is created. This is also configured on wiki, and this is the following system messages: