I would also mention Codewars, which currently has 28 tasks translated to Nim. It is not much, but it is twice the Exercism's 14 tasks.
I have just checked and translating tasks to a new language on Codewars seems fairly easy.
When I first started programming (in Python), I went through various programming practice sites, and what I remember from then - Exercism's setup was too high barrier for me to cross, so I decided to use Codewars and Checkio (which I found to be the best for me, but it only supports Python and JS, AFAIK), which were much simpler.
This could be a deciding factor for some Nim newbies too.