JSON files have been added to pywikibot/i18n, which now has python and JSON files with the same messages. The JSON files are not used yet, as the code changes to enable JSON have exposed packaging problems that are the subject of RFC https://www.mediawiki.org/wiki/Requests_for_comment/pywikibot_2.0_packaging
We need syntax validation of these JSON files for gerrit submissions, as message changes typically need to be approved quickly (and without errors) so they can be merged and the core & compat i18n submodule updated, before the messages can be used in code changes.