I would echo what some others are saying here as well; specifically what @sean recommends.
But it also sounds like you really want to deepen your knowledge of R rather than adding a programming language to your resume, so I'll try to answer differently than the good advice already added above.
In your question you mention Lisp, Haskell, and Prolog--it sounds like you're interested in a language that shifts the paradigm a little rather than a different implementation of C++ (I could be wrong). I don't know if you've already looked at R on wikipedia and looked at its 'influenced by' section, but it seems like there are two interesting ways to go--with
Also, thank you for asking this question because it gave me a chance to read this: https://www.stat.auckland.ac.nz/~ihaka/downloads/Interface98.pdf
Lots of things I didn't know before
He recommends this book for Scheme https://mitpress.mit.edu/books/little-schemer
I really tried going this route before using this book http://landoflisp.com/ but it is languishing on a directory somewhere. I still plan to learn some Lisp, just don't know when I'll do it!
Like @raybuhr mentioned, you can also learn Clojure which is a Lisp, but slightly different. It's also a bit controversial, as many are both for and against it. But there are companies that use Clojure a lot (I also think PNW is the Clojure mecca). Clojure can access the JVM (which is either a feature or a bug) and it has a statistical and graphics computing framework as well (http://incanter.org/). I don't know a ton about it, but when I thought about learning it I started here: https://www.braveclojure.com/
Another interesting connection to another language is with ClojureScript https://clojurescript.org/
My two cents Either of the languages above would be cool to learn (I would like to spend more time with both in the future). However, if Python at all interests you I think it's also a good option even though it's not really like R, S, or Lisp. Effectively, I think of Python as pedagogically useful for learning computer science and R as pedagogically useful for learning data analysis/statistics. Both were really important and useful to me personally. If this sounds good then I would recommend you use this site: https://interactivepython.org/courselib/static/thinkcspy/index.html rather than buy a book. I think it's really clear in explaining how the language works.
Hope that helps--I ask myself this question a lot and am so grateful there's a community here now to ponder ensemble