Android: Previously, we've featured Chrome extension Lingua.ly that allows you to learn new words in a foreign language. Lingua.ly's new Android app not only provides a similar service, but allows you to build your own personal vocabulary collection.
The app works very similarly to its desktop counterpart. You'll start by being quizzed on your current language abilities and the app will give you articles to learn new words based on your reading level. It also contains flash cards, quizzes, and a personal dictionary list so you can add words that you've either learned or want to learn.
Lingua.ly [Google Play Store]