In the same vein as T89948: Move magic word translations to JSON, namespace name i18n in core should be moved out of the Messages*.php files to a JSON format. In the same vein, *.namespaces.php files in extensions should die and be replaced by something in JSON.

Hopefully this will eventually allow namespace names to be translated from translatewiki.net (so long as translators are made aware of the consequences of changing a namespace name - i.e. breaking links on existing MW installations).