If you already make heavy use of Google Translate, this add-on can save a lot of time. Instead of having to open Google Translate in a new browser window, you can just highlight some text and translate it from inside the document.
After installing it, start it from the Add-ons menu. To translate your Docs content, let select some text, choose the languages and you’re all set then click the Translate button. You can then insert the translation back into your document.