localize the interface

[willkg]

I'd been avoiding localizing the interface because it required a bunch of infrastructure to do and I hadn't had time to do that correctly.

This issue covers:

• [ ] setting up the code infrastructure for generating messages.pot files,
• [ ] going through the codebase and adding gettext calls to strings,
• [ ] updating documentation to cover l10n conventions,
• [ ] setting up richard on transifex

[willkg]

I think I'm going to push this one off to the next milestone. It's important, but so is everything else.