language on releases

Greetings

currently the documentation states the following about language of the release:

```
language (string, slug): the primary language used in this particular release of the work. Only a single language can be specified; additional languages can be stored in "extra" metadata (TODO: which field?). This field should be a valid RFC1766/ISO639 language code (two letters). AKA, a controlled vocabulary, not a free-form name of the language.

```

1. for ISO639 if you want two letter codes ISO639-1 should be specified. ISO639 has 6 parts, the two letter codes comprise part one.

2. referencing RFC1766 is old form. [RFC 1766](https://tools.ietf.org/search/rfc1766) was obsoleted by [RFC 3066](https://tools.ietf.org/search/rfc3066) which was obsoleted by [RFC 4647](https://tools.ietf.org/search/rfc4647), which was obsoleted by [RFC 5646](https://tools.ietf.org/search/rfc5646). The stable way to reference this chain is to reference [BCP-47](https://tools.ietf.org/search/bcp47).

3. Is there a downstream technical reason to limit this field to two characters? instead of supporting BCP-47?

Start by reviewing the documentation passage quoted in the issue and the current definition of the release language field. Determine whether the ISO 639-1 and RFC references should be corrected and whether BCP-47 support has a downstream technical constraint; done means an agreed specification update or documented decision.

Written by the indexing model from the issue text.