Standard Rule of Thumb: As soon as you can complete the "hello world" tutorial, you should add the language to your resume.
What will you have achieved after you "hammer" them on that technology?
Will you genuinely be closer to assessing whether that candidate can do the job you need done?
I think that it is reasonable to ask questions about what is on a resume.
I told them I had been coding in C++ for four years.
They said "So what level are you, Expert or Guru?"
I thought it was a trick question, and replied that anyone who calls themselves an "expert" after less than 7 years or "guru" after less than 12, is probably over-representing their skill level.
Unfortunately (1) they were serious, (2) what I said applied to all three of my interviewers and (3) it was an internal interview. I had to leave the company very soon afterward. My former boss insisted that I was at least as skilled (at coding) as those three (I passed the technical questions comfortably), and it was just my reluctance to self-promote that let me down.
I don't regret it at all. I would not have done well in that culture, whether I had the coding skills or not. And I avoided having to move (1000s of miles.)
A better rule is to put a language on your resume when you can think of a question, and you immediately know how to approach (not solve) the problem using language X.