Bugs with Brazilian/Portuguese internationalization

[nickserv]

Root

GET / HTTP/1.1
Accept-Language: pt
Host: youmightnotneedjquery.com

1. HTML/HTTP language is not set
2. Language is English, should be Portuguese

Brazilian/Portuguese

GET /br HTTP/1.1
Accept-Language: en
Host: youmightnotneedjquery.com

1. HTML/HTTP language is not set
2. Language is Portuguese, should be English
3. JS error in console:

Uncaught ReferenceError: getInitialSliderState is not defined
    at HTMLDocument.<anonymous>

4. No interactivity due to JS error

[nickserv]

Once we finish #266 it should be easier to extract internationalization data into separate files with shared logic.