You can also click the extension's button and type some text you want to translate or click "translate this page".
The nice thing about the extension is that you can change your primary language in the extension's settings without changing your browser's interface language or your operating system's language. For example, Chrome for Mac "determines the browser interface language by the Language & Text setting in System Preferences."
"The Translate team is working hard to connect people by breaking language barriers across computers, mobile devices and Internet browsers. Our users make more than 1 billion translations a day," informs the Google Translate blog.