The language data used by mw.language is currently only for the wgUserLanguage (exported by ResourceLoaderLanguageDataModule.php). This limits the ability to add i18n/l10n support for gadgets and user scripts that are used for editing, since edits should be made in the wgContentLanguage of the wiki.
The language data (digitTransformTable, grammarTransformations, pluralRules, etc) are not accessible even using the API. An API endpoint should be created to expose this data.