Google Translate for Google+ stopped working a while ago due to continues changes being implemented by the company on the Plus platform.
Google developers have updated the app and it is now fixed for the latest updated Google Plus frontend.
Google Translate for Google+ comes very handy if you follow users from other countries. It adds a translate option to individual posts. You can also tell it to process your entire stream.
Google developer Josh Estelle added that they are working on a similar extension for Mozilla Firefox users too.