This looks like an awesome resource for learning new languages and also for understanding Lisp at a more fundamental level. I'm going to share this with my students next semester when I teach PL.
Are there any other projects like this that let you compare implementations using many different languages (e.g., Rosetta Code)?
It does remind me of the Real World repository [0]. It is basically a Medium clone implemented in tons of different frontend and backend frameworks. Probably not exactly what you were looking for, since it's more related to the frameworks rather than the programming languages, but it is still interesting nonetheless.
Are there any other projects like this that let you compare implementations using many different languages (e.g., Rosetta Code)?