As discussed in T115218, MediaWiki already supports JSON-based grammar transformations in languages/data/grammarTransformations.

LanguageLa still implements grammar transformations via convertGrammar() in includes/languages/LanguageLa.php.

This task tracks migrating those grammar transformations to the JSON format and removing the PHP implementation where appropriate.

Follow-up to T115218 @Nikerabbit suggested LanguageLa as one of the straightforward languages to migrate.