See slack thread for a little bit of backstory.

The main concerns we should revisit here include:

• non-consistent behaviour of the default I18NauthoringSupport (returns a default locale, but without a site configured - returns empty list of available locales, not including the default one).
• NPE-prone behaviour of the multi-site implementation: in case related site is not resolved for the passed item/node, null is returned for both default locale and available locale list calls.
• cumbersome #deriveLocalisedName API: for instance always requires to do isDefaultLocale check before (could be done internally in the #deriveLocalisedName).
• no argument-less API (when we want to access the default i18n confiuration, without any reference item).

To be compatibility module agnostic:

• create a new default impl in ui-framework
• create new JCR impl in ui-framework-jcr
• keep the current impl in compatibility module (might extend the default/JCR impl)

https://git.magnolia-cms.com/projects/PLATFORM/repos/ui/browse/magnolia-ui-framework-compatibility/src/main/java/info/magnolia/ui/framework/i18n/DefaultI18NAuthoringSupport.java#69