Day.js has great support for internationalization.
But none of them will be included in your build unless you use that.
You can load multiple locales and switch between them easily.
There's also a locale.json file contains the list of all supported locales in the root of each release.
More details on each of the parts of the locale bundle and the way to update or customize locale can be found in the customization section.
You are super welcome to add a locale by opening a pull request.