The list of 'what' to learn is the list itself, with the exception of the unknown unknowns which will become known unknowns when encountered. This to me IS learning. Still reads like a rant after being faced with more learning than hoped.

  - How do I install it?
  - Which editor? IDE? Vim plugin?
  - How do I.. read a file? parse JSON? environment variables?

How do I do any of the things that aren’t part of the core syntax/semantics but are super common problems people face every day? I’m going to have to memorize another 100 functions and their parameters, aren’t I?

  - What are the language quirks that will cost me an hour to discover?
  - How is the help organized?
  - I hit problem X.
  - How do I debug?
  - Testing.
  - How do I build? package? manage environment?
  - Package management.
  - So… the language community.

Is it worth it? For most languages the answer will be "No", for others unless you've already really gotten into several very different ones you won't know.