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.
If we're going to record languages (which I think is a good thing), then we should check that the language code is valid, just as we're checking that the country name is valid.  That means embedding a list of recognized language codes in the checker.