Add language field to YAML header

In https://github.com/swcarpentry/bc/issues/604 I had suggest that we track the language used at the workshops. This is the pull request for that and include:

- Change in `index.html`.
- Change in `CUSTOMIZATION.md`.
- Change on the validator.
- New tests for the validator.
Language should be optional, with 'en' inferred if nothing else specified.