Build an interactive app for Android and Ios

Hi! We want to develop an app to learn languages.
We’re in the early stages of this very profitable project and we’re looking for a Mobile App Developer.

The app will have to include the next features, among others:

Show texts, videos, images, etc.
Interactive language exercises and quizzes
In-app payments (will include premium features)
Ads

We will provide the content for each of those sections.
This is a mid to long term project, we want to develop a lasting professional relationship with our workers.