Leaning the second language is certainly hard, but learning your fifth or sixth language or so becomes easier? Hopefully? At least if you put forth any effort or interest I would think? If anything, I feel like the biggest trap people fall into is assuming they know a lot of languages when they really just know a lot of Algol-descendants... you should make sure to spend time really building something meaningful at least once in at least Haskell, Erlang, and a Lisp (and a lot of others, but if you haven't worked in those three... have you really programmed? truly?).

My story: I have been programming since I was a little kid, having used mostly stuff like Logo and BASIC for years; after spending a bunch of time with Visual Basic I kept failing to learn C. In high school I took a course on some slideshow programming system whose name is escaping me right now, and then one on Pascal. OMG Pascal was hard for me: I made flash cards to try to learn the keywords, and I was struggling. There was something concurrent I was doing with simple JavaScript, which seemed to be going OK-ish.

I then started taking a second course in Pascal--an AP class--but by this point it had all clicked: I had been programming at that point--even writing software for local companies sometimes (in Visual Basic)--for like 8 years and I finally got it. The teacher noticed, and since I was only a junior he said I shouldn't bother continuing in Pascal: I'd help him learn C++ and co-teach the class the next year with him, and take the AP test in C++.

That was... over 20 years ago now? I now feel like I can just learn any language almost immediately. I remember being in the audience when Apple announced Swift: I was looking at the slides and thinking "ok, I see what they are doing here: this is like Objective-C crosses with Scala but with some of the syntax of Ruby... I bet I could program in this". I spent the next day playing with it and had already befriended the Swift compiler people as I was finding tons of bugs, and then I gave a talk on Advanced Swift Internals two days later across the street at AltConf.

I personally feel like this kind of mental process is "teachable". I thereby sometimes have gotten to teach a course at UCSB (I have sometimes been hired as a "1/8th lecturer" or something like that to do this) on programming languages, where I try to take students through the breadth and depth of syntax and semantics, rather than just focusing on a couple examples. Sometimes we look at a semantic and how it evolved, sometimes a syntax and how it got reused, and sometimes a use case and how various languages tried to support it. By the end of the course I expect you to "know" none of the languages, but to hopefully be able to quickly use any of them with some examples. The big project for the class is to design and implement your own "esoteric" language.

FWIW, I "thereby" personally am still in camp "it doesn't (exactly) matter"... as long as it inspires the student! Which I actually think is what this author is kind of getting at (and so I actually do agree with them) with that paragraph about "what about a student who X"... a lot of teachers seem to think "learn this toy thing and you will be able to eventually learn anything", but the toy is boring and doesn't do anything the student wants to work on and by the end of the entire experience you are lucky if they did any of the work at all. It is like teaching someone to play some horrible sounding musical instrument using example pieces that the student doesn't enjoy listening to with the goal of establishing enough musical theory and experience that they can learn a second instrument / style some day and eventually get good enough to do what they wanted. It is just unrealistic? I was able to get as good at the languages I was able to largely because they were useful to me... and honestly, that Pascal wasn't was probably part of what made it so hard :/.