Starting from version 7.0 (Nougat) Android offers the ability to select multiple languages which enhances the experience for bilingual users especially those who prefer languages other than English.

However some manufacturers have not implemented this functionality on their devices for example Xiaomi (MIUI 10) and Oppo (ColorOS 5). This app allows users of such devices to select more than one language. In addition just like any other language-changing apps it allows users to add unsupported languages.

Normally only system apps are allowed to change the system language. Therefore you'll need to grant this app special permission using a computer.