There are two FIDL services providing this functionality. They are intended for different use cases, however, so it is important to pick the correct one to use when in doubt.
This is the read-only access to the internationalization profile. Depending on the realm, it can provide per user, or system settings, as appropriate.
This is the read-write access to the internationalization profile. Connect to this service when you are building programs that need to display and modify the internationalization settings.