In a custom MediaWiki installation, in the upper pane where Labels are listed, there is a link named Configure that points to Help:Extension:Wikibase/Configuring_languages.
This page should explain how to set the default languages shown into that pane, but the page completely fails at this. In the chapter "Changing the defaults" there is some generic notes on how to improve language recognition for WikiData, but no helpful information on configuring custom WikiBase.
Can someone provide the actual explanation on how to do this configuration? Or point to proper documentation if it exist already?