Language fallbacks for extended language subtags

[jurgenbelien]

A language tag consists of a primary language subtag and optional extended language subtags, ie. nl-BE, or en-GB. (See https://en.wikipedia.org/wiki/IETF_language_tag)

A site owner might want to have certain content localized for a region, based on the extended language subtag. A site owner might also not want to bother with localizing all content for all extended language subtags.

Currently they would encounter two problems:

1. We are not using fallback locales for fetching content where the specified locale is unspecified.
2. We cannot fetch a fallback locale for an indvidual record when using filter: { eq: $slug } in our GraphQL queries for individual pages records, because filter will not find the slug for the specified locale.

Solutions might be:

• Add the fallback locales to graphql queries.
• Add a retry to datocmsRequest with the fallback locale to fetch the primary language if available.
• Fetch content from Dato based on ids instead of slugs, and use content collections to map the slugs to ids

[jbmoelker]

Could be solved with #278