What's the best language/IDE to learn Android and iOS development in? I have absolutely no experience but I'm looking for something new to try

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C# is the way to go if I'm not mistaken. My friend develops apps for Android with visual studio and Android studio and he uses c#. I work with game engines so I wouldn't know for sure

I use android studio, but if you want crossplatform, go for xamarin c# or appcelerator js

I recently began using React-Native and it's really nice, so if you're new to JavaScript it could be fun to give a try.

Classic answer: Android java, iOS swift
Modern answer: you can reuse a website code for building an app using one of a few:
1. Xamarin, C# .NET
2. ReactNative in Javascript
3. Less popular but I'd put bets on its future is NativeScript, why? Cause it is used with Angular which is proprietary to Google (and Microsoft also gives a hand there)

There are a few weaker options like Ionic2, Cordova, etc. They're dying so I wouldn't go for those (they use slow and not full APIs)