Add translations for one other language

[jace]

In the future, we'll need our apps available in languages other than English. i18n is really hard to retrofit into an app, so we need to plan for it upfront, and continuously confirm it's working.

HGTV should be made available in at least one other language as a test case, as it contains very little UI text and should be easy to translate. The tasks:

1. Use a framework like Babel to manage translation strings (Baseframe uses it).
2. Ensure server-side translation for text originating in Python code or Jinja2 templates by using the _() and __() wrappers (Python) and {% trans %} tag (Jinja2) wherever UI text appears.
3. Use format_* and ngettext wherever numbers or singular/plural-based text appears.
4. Pick an i18n framework for JavaScript and figure out how to do client-side translations (this should be solved in Baseframe and used here).
5. Commission a translation into at least one other language that the dev team can understand.