Weblate has started as a translation system tightly bound to Git version control system. This was in no means design decision, but rather it was the version control I've used. But this has shown not to be sufficient and other systems were requested as well. And Mercurial is first of them to be supported.
Weblate 2.0 already had separated VCS layer and adding another system to that is quite easy if you know the VCS you're adding. Unfortunately this wasn't the case for me with Mercurial as I've never used it for anything more serious than cloning a repository, committing fixes and pushing it back. Weblate needs a bit more than that, especially in regard to remote branches. But nevertheless I've figured out all operations and the implementation is ready in our Git.
In case somebody is interested in adding support for another version control, patches are always welcome!