But why? Out of the box you have build-in backends for Django Template Language (and Jinja2 as an alternative). Django’s own template system is really powerful, and for many years it was more than enough in terms of building websites.
Virtual environments can be described as isolated installation directories. They allow you to localize the installation of your project’s dependencies, without forcing you to install them system-wide. You can have multiple environments, with multiple sets of packages, without conflicts among them. This way, different projects’ requirements can be satisfied at the same time.