[lmm]

Do a real project - making a tool someone (possibly you) actually wants to use - in the language. That's the only way to genuinely learn/evaluate.

For Scala (and I guess also Kotlin and to a certain extent Python), you have the advantage of easy JVM interop, so you can use them in an existing Java project. That's how I learned Scala (which I'd recommend): start by writing just a couple of classes (a small helper application with a main() where I wanted to invoke some of my existing Java code), start by writing very Java-like code, and then gradually work your way up. Don't try to force the fancy techniques before you have the problems they solve and can understand what they're good for. It took probably 2 years before I wrote any code that used (generic) monads, another year before I used any monad transformers, another year before I wanted to use final tagless...