Yggdrasil Flarum is a forum after all, so each tag on the front page is more or less a category itself
So, if it is meant to be a category, why didn't the Flarum-dev's call it a category then?
My first reaction after i installed the german language pack was "Oh, it's falsely translated". And i looked specifically at Tag/Kategorie because i know that dilemma through my professional background. tag and category are similar and sometimes used as synoyms. But at it's core a tag is a "Thema" and a category is a "Kategorie".
Don't get me wrong Yggdrasil, i perfectly understand your point and i can live with the "Kategorie"-translation, because - as you said - it makes a little more sense in the forum-context. But imho it's not correctly translated and maybe it should be changed to category in the basic/core english version if this was the intention.