Language negotiation doesn't respect langcode filter

Problem/Motivation

JSON:API behaves oddly when retrieving translations or trying to filter by langcode

In this example:

• http://localhost/de/jsonapi/node/blog returns German nodes, and English nodes for which there is no available German node (which we don't want for this site)
• http://localhost/jsonapi/node/blog?filter[langcode]=de Filter is completely ignored and only English translations are returned
• http://localhost/de/jsonapi/node/blog?filter[langcode]=de Returns only German nodes (which is what we want)

This bug was found by @alwaysworking → who may have some additional information to add.

UPDATE: The above behaviour was on Drupal 8.6 & 2.1 of JSON API (See #5 🐛 Language negotiation doesn't respect langcode filter Active )

Currently on Drupal 9.1.x:

• http://[site_url]/jsonapi/node/[content_type]:
  Returns only nodes in the default language
• http://[site_url]/jsonapi/node/[content_type]?filter[langcode]=[langcode]:
  Returns only nodes in the default language for which there is an available translated node in language [langcode]
• http://[site_url]/[langcode]/jsonapi/node/[content_type]:
  Returns nodes in language [langcode] if there's an available translated node in language [langcode], and nodes in the default language for which there is no available translated node in language [langcode]
• http://[site_url]/[langcode]/jsonapi/node/[content_type]?filter[langcode]=[langcode]:
  Returns only nodes for which there is an available translated node in language [langcode]