The app will hang on startup if the app has been previously exited when using the flashcard module.
The bug happens only on iPhones and iPod touches.
A fix is already posted to Apple and is waiting for validation.
Meanwhile, it is possible to use Wakaru by resetting the app. All you have to do is to go to iOS settings and turn on the "Emergency recovery setting". Then it is OK to relaunch Wakaru.
UPDATE : the update is validated and version 2.1.0 is available