Well, the preferred language for Android is Java, although you can use others. For iOs, it was objective C, until they invented Swift.
For browser-based work, I would strongly recommend AngulrJs over plain old JS.
But, my answer is don't develop native apps.
Leverage your existing skills. Look into Cordova from the Apache foundation. It is a free framework that let's you code HTML, CSS & JS - skills that you already have.
This will, of course, be cross-platform, which is a good thing.
The major point of Cordova is that it uses JS libraries to let you do what browsers normally can't. With it you can access a device's camera, GPS, microphone, address book, etc, etc.
Visit their web site & I am sure that you will be convinced (don't forget to look into AngularJS too).