Mediawiki is moving from PHP array based localisation file format to json format.
This is based on the RFC: https://www.mediawiki.org/wiki/Requests_for_comment/Localisation_format
A lot of extensions were also migrated to the new localisation format, thanks to Siebrand Mazeland
The json based localization file format was first introduced in our frontend javascript i18n library https://github.com/wikimedia/jquery.i18n
If you are interested in seeing some of the sample json files see https://gerrit.wikimedia.org/r/#/c/122787/ , claimed as “largest patch set in the history of MediaWiki”