Figure out i18n strategy for spec text

Splitting this off from #330.

We'd like to enable translations of the spec into other languages in a scalable (ie. crowd-sourced), convenient (ie. minimizing manual synchronization work), high-quality (ie. community-reviewed and supported) way. [Crowdin](https://crowdin.com/) has been successfully used by other projects with FB involvement (eg. [Jest](https://github.com/facebook/jest), [Yarn](https://yarnpkg.com/en/)), although I am not sure how easily it can be adapted to our Markdown-centric workflow. Let's investigate.