Sorry if this is a dumb question, but I do not find any possibility to display the whole category-tree of a wiki (not just all subcategories of a certain category). Is that possible?
Topic on Extension talk:CategoryTree
No, unless all categories have a parent category (except that parent category itself), in which case you can use that parent category and everything else will go below that. Of course you'll need to interact with the tree so it loads subcategories by clicking on them. Displaying the tree at once is probably not doable because there may be loops in the tree.
@Marcreal: this is possible in MW 1.27 and corresponding extension version if you use root category of the tree. Any higher version of MW and CategoryTree extension is messed up. I have no idea if it is due to core changes or extension. It messes up the trees on en:WP as well but either they did not notice or do not care.