I guess that's really personal. I've also thought about this exact same question, a few years ago when I wanted to start with mobile development. I choose working with a native language because I wanted to dive deep into the OS's possibilities and work seamlessly with the language and IDE of choice. I choose Swift because I really like Apple's products and UX/UI.

The learning path actually took a year, before I could start working professionaly as a iOS Developer. I didn't regret a single decision I made, I love my new career. Mobile development is alot of fun and it's challenging to learn all possibilities and be good at it.

I would go for one language, get really good at it and move to the next. I'm learning Kotlin as we speak.

Personally I wouldn't go for anything other than native, but I'll save that discussion for another day.

Another tip I would give is, when you start learning, give it ALOT of time and learn each day. Read, try and go through the first big hurdle. When your mind screams; I can't do this - yes you can. Go again tomorrow and keep learning. Eventually, everything will fall into place.

tl;dr - what OS do you like best? Which do you personally use? Go for the OS you like, and love to use. But if you ask me? Keep it native. So Kotlin or Java for Android and Swift for iOS.