I find the limitation that all descriptions must be plain text to be incredibly limiting. I often want to link related articles in descriptions.
Apparently the German Wikipedia has figured out a way to have the descriptions rendered as wikitext ... I'd like to know how to do the same.
No, German Wikipedia figured out a way to have descriptions rendered as wikitext on the template documentation page – the actual TemplateData (used by VisualEditor etc.) still can’t contain links. See de:Vorlage:TemplateData#Verbesserte Präsentation for how the template documentation page and the actual TemplateData differ.