It depends what language you want to learn, as there are different apps on the market for different languages.
That being said, the hello-hello language apps are good (I've only had a brief go!), they do many different languages (Spanish, English, French, German, Chinese, Japanese, Russian and many more).
They are free (to a certain extent), have good ratings and follow a research based methodology; Developed in collaboration with ACTFL - The American Council on the Teaching of Foreign Languages.
Android Market: https://play.google.com/store/apps/developer?id=He...
hello-hello website: http://www.hello-hello.com/
Hope that helps!