Add support for multiple languages in video output

[joedolson]

Need to be able to output the additional languages available.

Tasks:

• [x] Output multiple versions of captions
• [x] Settings for target languages
• [x] Settings for any language
• [x] Tagging of posts by languages available
• [ ] Have description and title available in languages

[joedolson]

Partially completed in https://github.com/wpa11yday/conference/commit/5dcc56fb6b59313a572ea9402ede4769b88bbb1b

[joedolson]

Taxonomy present; needs design, see #60