Improved multi-language support

[mDuo13]

Much of the current multi-locale stuff could probably be automated by making certain assumptions. This would take some investigation to make a proper plan first, especially one that would not be too much migration work, but here are some obvious ones:

• dactyl_build could compile .po files, if present, by calling Babel automatically
• Similarly, it could automatically extract {% trans %} strings to update the .pot file and .po files when run. If possible, it would be good to have a notification that new strings need to be translated.

Bigger challenges include:

• Support multiple languages in a single "target" with separate prefixes based on the language? Or build multiple "targets" (one per language) with a single command?
• Managing hyperlinks in MD content (including translated content) so that things go to the correct translation when using absolute URLs
• Maybe impossible, but some kind of assist for managing anchor links in translated content & links to/from untranslated pages.
• Guidance for managing translation age / updates. Maybe with Transifex? TBD.