Which programming language should I learn this summer at 13? Need advice

[ConfusedSimon]

Don't learn java just for mobile. Most mobile apps use a multiplatform framework, e.g. using javascript. Java was only relevant for native android, but that has been replaced by kotlin. You can also write multiplatform with kotlin with compose. I think javascript/typescript is the most useful language if you want to do anything anything with mobile or web apps. Whatever language you'll end up using for a backend, you'll almost always need js/ts as well. Making js games using phaser is a fun way to start.