rails/rails is “the” Rails repository. It’s where people commit code into Ruby on Rails.
rails/docrails is a copy of rails/rails. This lets people to push documentation patches (write access for which is doled out by the core team) without the need to send in pull requests. We merge rails/docrails into rails/rails and vice versa once in a week or a fortnight. More on this can be found at http://weblog.rubyonrails.org/2013/6/14/docrails-back-to-the-roots/
I think we don’t want to maintain localized copies of the guides in the repo, but I’ll let Xavier (fxn on github if you’d like to ping him) answer this.
Reason is we strongly believe docs have to be updated alongside code. A complete patch has code, test coverage, and update affected docs. Is the way to keep your test suite and documentation in sync with the code base.
And in practice that is only going to work with English.