If you want to view your learning achievements, you have to:
1. Tap on the "You" tab.
2. Go to "Stats".
3. Tap on "See achievements".
4. Tap on the dropdown menu icon to see all your achievements in a category.
Note: Your unlocked achievements are fully colored, while the locked achievements are only outlined. Achievements are unlocked by reaching certain learning milestones. At the moment, this feature is available only on Android devices.