Arc will detect when a page is not in the default language and a translation prompt will appear in the URL bar. To translate the page click the 'Translate' button that appears at the top of the Sidebar:

You can also right click on any page and select the translate option from there:

To undo a translated page, head to the ‘Translated’ button in the URL bar. Once there, click on 'Translated' and from the drop down menu that appears select ‘Use Original’ to go back to the original language the page was in.