Learning a new language

Almost two years ago I blipped twice about the MIT App Inventor. A tool that would help me create an app for my smartphone. Well, it did, but only to a certain extend. When I had the need to develop it further I was held up by some things. First of all I thought the UI (user interface) too complicated. And secondly the logic I was trying to code was not so easy to implement in this tool.
So, I have now moved to Kotlin, the so-called "First Programming Language" (at least as per Mr. Google). There is a lot of documentation available on the web, Google has provided an IDE (developing environment), and free online courses are also available. I am actually busy with two separate training plans.
The book cover is of a 800+ pages book, for when I want to read details, and as reference. Sometimes a book beats a screen.
Now let's see how far I will come, learning yet another language. Can I add it to those that I master with at least 80%?