ReDJ have a plugin that must be enabled to do the work, and a component that lets you manage redirections. You should create an item like my example for each Google's error page. Your proposal:
From URL: ^/en/$
To URL: http://{siteurl}/a/en/
Will not work because with current ReDJ release, source URL is dynamic (you can use regular expressions), but destination URL is static, so all matching URLs will go to the same destination.
I plan to introduce dynamic destinations in the next ReDJ release.
Regards,
Luigi