If you are using JavaScript and/or a framework like React, Vue or Angular, I suggest you go with Node.js. You will be able to use the same programming language (JavaScript) for your front-end and backend. If you are using something different for your front-end and your app is not a very high processing app, Node.js will work just fine. If your app requires a lot of processing (games, etc) Java will work best.
This is the first programming language I learned. After doing great research about programming languages I decided to learn Java since it has a great community supporting it and it's a cross-platform programming language. It is widely used and knowing it will give you high chances to find a job in the industry.
If you're looking for a simple to-integrate CMS, Contentful is a good option. Their free option is quite useful so you don't have to pay unless the website requires a lot of content, assets, etc. To connect to it you just need to make a request from the website and decide which content you want. I believe it uses graphql for getting the content.
If you want to try Android development the more 'android' way you can do this is by using Android Studio. Flutter is a cross-platform solution similar to React Native where you write your application once and the code you write (in this case Dart code) is able to run this app on android and iOS environment. That's the magic that cross-platform solutions offer. Using the native solution (Android Studio) gives you the full Android experience. You can choose Java or Kotlin when developing on Android Studio. If you want to build an app for android and iOS then Flutter is your best choice.
