Adding custom ISO Code for language dialects

General
1

In German we have many dialects and subdialects etc. I want to build a system for a linguistic use case, were content objects should be translated into dialects. Therefore I would like to add custom ISO codes to the ISO Locales JSON File.

Does anybody has an idea how to do that?

2

The only reasonable approach seems to be editing the JSON file and making this change reproducible with patch-package. I documented the approach here, for anyone who has a similar requirement.