Answer 1

A coding language. You might like having a good background of Kotlin and know about Android studio to develop your app. Then know about Swift language because you will want to consider your users who have iOS on their phones.

You might also have to look at UI and UX for your app to be 👌

Then again it won't hurt to know languages like Python which can do the app's scripting functions and some bit of AI so your data from the app's users is organized and protected.

PS [Off-Topic]Learn your competitors and do it different.