There are many frameworks out there that allow you to use HTML/CS/JS to build apps.
Java isn’t really difficult to learn. I’m in my mobile development course and I already learned Java and we use Android Studio to create the apps.
You start learning Java and that leads to other opportunities as well. Some of my colleagues use Eclipse and NetBeans for their apps, and we use those for Java, and some for PHP.
But asking “is it worth it to learn a new language,” I ask why limit yourself? Java and the C family have many similarities. The thing about programming is there are many tutorials and places to get help if stuck.
I personally have knowledge with HTML, CSS, JS, Java, XML, Freemarker, some C#, C++, close to being a Windows certified administrator, and have been looking into network administration, ethical hacking, and data analytics.