This is a Greasemonkey script. If you don't know what Greasemonkey is? Follow this link http://diveintogreasemonkey.org/install/what-is-greasemonkey.html

Hold the Alt key and double click on some text (to select it). A nice translation popup shows up! Supports many languages too!

There are other scripts that do that like highlight2translate and TrannslateText (both are found on this site, off course) but there are limitations.

With this script...

- You can change the language on the fly. - You need to hold Alt for the popup to show up, so it is not annoying!

Install it here: http://userscripts.org/scripts/show/59032