Except for maybe Haskell[0] and Lisp[0], I've begun to feel like there isn't any language that I can't quickly learn with how I'm able to google at the moment.
For example, I'm currently helping some fellow students with their F# assignments, even though I don't do F# at all. I just know what to google, because I know the concepts that I need translated to the specific syntax.
[0] Maybe Lisp, but Haskell is way too different from other languages in the way to programme. I'm currently learning it, but it's by far the language that has taken me the longest.
I remember how I was starting to think at the beginning of college, that with a reference sheet I could pick any language and start writing in it ... you know, C++/Java/Python, it all looked the same.
Then I applied for a course in Programming paradigms, that boiled to 4 weeks of haskell and then 4 weeks of prolog.
So, definitely add prolog to your list :-)
It was eye opening :)
I have an opposite sort of experience: I can't learn things fast enough if they aren't mathematical in nature. I used to think programming was disgusting because of Python(heard Java is even worse) till I came across Clojure. I immediately appreciated how the latter imitates pure math so well :) Never going back to Python. Once in a math mindset, learning anything imperative is painful.
Or any other concatenative or stack-based programming language. And Agda and Idris and other dependently typed languages I feel deserve to be on this list. There's also Qi/Shen. Of course various assemblers, too. And logic languages, like Prolog, Datalog, kanren. And then...
In short: every time I thought I know "most" of what there is to know about something I soon discovered that I was wrong, and that "something" was a vast area which I didn't really explore at all except for a tiny little fraction of my immediate surroundings.
I think there's only two and Lisp doesn't need to be on that list, but maybe I say that because I've been using Common Lisp for so long.
Lisp ought to be easy since all you basically have to learn is: "(function parameter1 ... parameterN)".
Coming from Lisp I've been trying to learn some Haskell and holy cow syntax[1]! There's a lot to parse and know (compared to Lisp) before being able to write a small program.
[1] I know it is not literally syntax but mostly function calls, it's still a lot of extra special characters one needs to know about.
For example, I'm currently helping some fellow students with their F# assignments, even though I don't do F# at all. I just know what to google, because I know the concepts that I need translated to the specific syntax.
[0] Maybe Lisp, but Haskell is way too different from other languages in the way to programme. I'm currently learning it, but it's by far the language that has taken me the longest.