Google chrome has in built translation feature, that allow us to translate from any web language to our preferred language.
Then why you should download an extension of Google Translate ?
Google chrome has in built translation, that does the translate operation for the whole web page where as the Google Translate extension offers, you to select text from the web page, click the small Google Translate icon and get the translation done instantly....
Even you can copy the text from the web page and click the google translate icon from the menu bar and paste the copied text to know the translation results without visiting the Google translate website.
The most beautiful thing of this extension, that you can set your primary language in the extension's settings without changing your browser's content language or your OS (operating system) language....
Google Translate extension Link
Sources : Google Official Blog