This app encourages you to learn Chinese each time you unlock your phone's screen. This is how the creator's described it to me:

Unlock Your Brain now offers seven different languages in the app, including Chinese, English, German, French, Portuguese, Spanish, and Italian. Users can combine any two languages - as source language (user speaks source language) and target language (user learns target language) - according to their needs.

For learning Chinese, users can choose Chinese as target language and their mother tongues as source language. We offer different vocabulary packages and they can select the one which fits their level.

Then learning starts on their mobile lockscreen. When users unlock their phones, we will teach them one word on the lockscreen, they select the correct meaning/translation of this word in order to unlock and access their mobiles. There is also voice function, they can hear pronunciation if they want.