Crib sheets
Why crib sheets? Like any programmer I have used many languages in my career, starting with C, then C++ before moving on to Java, Javascript and Python. Along the way, I have picked up and used a variety of additional languages when needed, including C#, Lisp, Scala, Clojure, Prolog, Groovy, Haskel and Ruby among the list.
What I have also found is that once you are a seasoned programmer and know 1 or 2 programming languages it doesn’t take much other than understanding the syntax and the specific language idioms to get up and running and the quicker you do the quicker you start to be able to write something worthwhile,
Crib sheets are my way of learning a language, and they are also useful when I have to pick up a language after a period of non-use. I start with the basics and create a list of all the syntax and idioms that I need to know up to an advanced level. I can check back if required, or quite often have them open in another window as I code.