Internationalization

Long-term, Sapphire needs to support i18n for non-English locales. I think the best way to proceed is to use .po files (like mainstream desktop free software), along with the po2json utility to get it into a form JavaScript can process. From there, the support code for i18n is like 3 lines for what we need 😛

I'm willing to work on this, since I have experience with i18n for purple-discord and can do a reference Spanish translation.

Depends on #4 (closed) for language selection.

I'd encourage _ syntax (and we're not using underscore.js and never will be if I have my way, so that's free :P), but I don't want to bikeshed that.