On a login box on auth0, you can set a default language, and other allowable languages for the tenant.
In order to pick a non-default language for a particular authorization, you need to pass a value for ui_locales to the Authorization Request Endpoint.
See:
https://auth0.com/docs/customize/internationalization-and-localization/universal-login-internationalization#language-selection

Unless I'm misunderstanding the documentation for the auth0 YAML file, there is no way to pass that parameter on through the auth0 R package's options.