Although having such a plugin for translation does not really serve any better purpose like more content for search crawlers to crawl. But it indeed helps your audience in case of a language barrier.
To add a translate option in your Blogger website go Blogger.com and select your blog.
Go to Layout and click on Add a Gadget where you want the Translate option to appear.
From the list of Gadgets select Translate and add it to your website.