What's the best coding language to start learning for building an app if you're an absolute beginner?
This is a tricky question, because any language will have its benefits and drawbacks.
I would start by thinking, "What is the platform for the flagship version of my app going to be?" If it's web, use JavaScript. If its something else, use a language that's popular on the platform.
If you're not sure, I would start with JavaScript. Every device with a screen nowadays has a browser, so JavaScript is highly flexible. Once you've got some more experience developing, it will be easier to see the benefits and uses of the different languages out there.