Default language change Drupal 8

We have a Drupal 8 site which has content translated into various languages (English, German, Italian) with English as the default language.

We have a use case where we need to change the default language for nodes which are already translated. For example:
- A node has content in English (default), German and Italian.
- We want to retain all the translations for that node, but change the default language ("Original language" in the translate UI) from English to German.

We have tested changing the site default language but this does not affect the default language of existing nodes. So we are anticipating that this will be delivered as a custom module that we can install on the site. There does not appear to be an existing hook for this so the module may need to directly update database values. We're looking for your advice on this.

We would ideally like a UI as a Views Bulk Operation or similar to allow us to select which nodes we perform this on.