Prussian language. The language is being revived by entheausiasts and, they are of the opinion that a MediaWiki project in Prussian makes a difference.
Translatewiki.net has its own requirements for new languages and with an ISO-639-3 language code these requirements are met. This allows for languages like Kotava as well.
Once the LocalisationUpdate extension is available to such projects, people can maitain their localisation at translatewiki.net and all the MediaWiki installations that have enabled their language will benefit from improving localisations.