More info about how the selection of languages for users currently work in Wikibase:
Termbox is showing languages considered "preferred by the user" (T213720) in the "more languages" sections.
These languages are sourced from their babel box (config.get( 'wbUserSpecifiedLanguages' )) or ULS (uls.getFrequentLanguageList()) - the latter being influenced by e.g. the country you are surfing the web from, user agent languages, languages previously used on mediawiki projects. They can, apparently, contain language codes that do exist but are not considered to be full fledged MediaWikiContentLanguages (wb terminology) but delegate to another language code instead.