EDIT October 2020: Sadly, Tinycards have disabled the app. More info here.
In July 2016 Duolingo launched a new app called Tinycards but it was only available iOS. On 2nd March 2017 they launched the website version which is also mobile friendly, so I excitedly checked it out, as it means you can now use the cards on any mobile device. Have a look (unless you already know it all from your iPhone ☺)
Edit September 2017: You can now also install an app on your android phone: Tinycards by Duolingo: Fun & Free Flashcards