That sounds crazy to me. If you throw me at a new language, sure learning the language might only take a couple of weeks (if it doesn't radically depart from what I already know), but learning the libraries/ecosystem is at least 6 months. Probably more for something yuge like Java or .NET.

I'll be referencing the doc and tinkering every little thing for many many months, which is sure to affect my productivity. Append to a file? Make an HTTP connection? Serialize a data structure? Have to look it up.

Then come all the 3rd party libraries a seasoned developer will already know. How do I talk to Postgres? Which of these five 3rd party libraries should I use? Which library do I need to help generate my SQL statements?

It's like you're looking for a journalist for an American newspaper, and you decide "oh, you don't have to speak english, you can learn that later". Sure, you can do this for an exceptional talent, but on average you're better off finding people who already speak english.