This means that the current "custom" extra languages we have on Wikidata and Commons will be available on every Wikibase as a Term language.
This is convenient for use because it means we should be able to render Terms (e.g in Lua) in languages that are not known by the rendering wiki.
This could also be useful in a federated properties context since if both Wikis are on the same version of Wikibase then we know that these extra languages will be available.