Just in my own experiences at work and with friends, I've found the opposite to be true - people pick up the language to use the framework and haven't met anyone that knew either language before choosing which one to use. Plus both frameworks (especially Rails) have a history of encouraging people to take that route as well.
That also partly explains why I started a Ruby tutorial ages ago and never got around to finishing the series, even though I still get pretty good googlejuice from it:
http://alabut.com/nonsense/2005/03/ruby-self-tutorial-day-1-...
So true.
I actually went the other way around and used both Django and Rails as means to learn (or get deeper) in Python/Ruby languages. Both frameworks do a great work on abstracting bits of the language, which IMHO made it easier to start (I tried getting Python a couple of years before but gave up since I didn't have a good 'hello world' idea to keep me motivated - being able to build some useful web app also helped, when learning along with the framework)
Rails -> If you know Ruby
Django -> If you know Python
I've been using Django for a while now, but the more we evolve our site, the less of Django we are using. Luckily, the separation of concerns thing really does work and we are able to replace components as we need to.