Works as soon as you activate it on your site. New visitors only see a small Translate button in the upper right of the screen. If you have WP super cache active, you may have to turn it off for a bit and clear the cache. Although that plugin speeds up your sites display, with it updates changes to your site can take a while to show up.
One nice thing about this plug in users can set this site to come up in their language and it will then default to their language on return trips.
I am installing this one on all my sites.