When you select words on a Facebook page and press T (SHIFT + t), Facebook Fixer can use Google Translate to translate that text for you (you can choose the language to translate to in the configuration screen). Right now only Firefox/Greasemonkey can show the translated text the way the screenshot shows it - other browsers will open a new window with the translated text.
Facebook Fixer is a free script for Greasemonkey (a Firefox extension) that customises and adds new features to Facebook.