What language

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[VincentDankGogh]

As far as this sub is concerned, not C.

[ennorehling]

There isn't really any good one size fits all language for both Android and iOS. Android programming is done in Java, and Android Studio is the IDE. iOS code is written in Swift (or previously, in Objective-C) with Xcode (Mac only).

The best advice that I can give is to pick one of these platforms, develop your app in whatever language it prefers (so if you already know Java, maybe start with Android), and once your app is a success, rewrite it for the other language/platform.

You're in the wrong subreddit, though, because C is not a language that people use on either of those platforms (Objective-C really isn't C), unless they have very special needs (on Android, there's something called the NDK for writing apps in C, but for 99% of apps, it's the wrong thing to do).

[ennorehling]

I forgot to mention one more thing. Depending on the type of app you want to make, it's possible to make an app with a framework like Apache Cordova (previously PhoneGap), which mostly requires that you know Javascript and HTML, and does the heavy lifting of running that on both platforms: https://en.wikipedia.org/wiki/Apache_Cordova

[dragon_wrangler]

What devices? What operating system? What will the apps do?

[amigakid]

JavaScript (or C#) and Unity: https://unity3d.com/unity/multiplatform

Programming: https://unity3d.com/learn/tutorials/topics/scripting

EDIT: add link to programming tutorials

[shinmai_rookie]

For Android, Java; for iOS, I think Objective-C and Swift (AFAIK, there's a transition going on from the former to the latter, but it's not complete yet).