Render enrollment emails in the student's language

**Context:** Course instructors have the option to enroll, unenroll and assign the "beta user" role to a list of manually-written email addresses. An email is then sent to each address to inform the address owner of the change.

**Problem:** the emails that correspond to existing users are not written in the user's language, but in the instructor's language.

**Proposed solution:** render the email messages and subject using the user's language, if available. If no user corresponds to a given email address, then the instructor's language is used (which is the current default behaviour).

This pull request is related to [this](https://groups.google.com/forum/#!topic/edx-code/ZVcwOq_vSRk) Google groups conversation.
I would like to see a test that tests the language=None case.