How do you learn a new language as fast as possible?

> How can I learn Italian as fast as possible?

> How can I learn to play the guitar as fast as possible?

> How can I learn to submit jiu-jitsu black belts as fast as possible?

For some things in life shortcuts are simply not possible.

To get fluency you will have to: write a lot of code, read a lot of code, work on other people's code, have other people review your own code.

One thing that helps is already knowing a lot of other languages :). I usually duck-duck-go stuff like "$language for programmers" and "$language in 20 minutes" or "$language for people who know $similar_language" in order to find guides/howtos oriented a people who already know how to program. Because reading a Language guide that also teaches you what is an if statement or what is an exception is extremely boring to me.

Get a job where that language is the primary one. Coding it 8h+ a day gets you up to speed.

Depends on what I need to do. Learning enough to read some code takes less time than learning enough to fix bugs. Getting to sufficient competency to write code from scratch takes longer, and leading a team and making architecture decisions will take longer still.

I can usually read any C-family language without any study, because they are much more alike than different. I will need some reference material to fix bugs and make small changes. I may need a tutorial or a non-trivial code base to study to get to competency. Before I would lead a team or make expensive decisions I would want a couple of years experience with the language and ecosystem.

There aren't any shortcuts.. But when I was in college we studied abroad and I learned way faster having to communicate than I would have in the US. So my advice would be to visit another country that speaks the language for an extended period of time.

Move there, no phone translate tools. You will learn because you have to.