Pick a Wikipedia and paste the articles you want to look at — or categories, in which case the articles inside them are used. For each one, the tool reads the same article in other languages and lists the categories they file it under that yours doesn't, translated into your language.
Every article costs a handful of API round-trips (Wikidata for the sitelinks, each sibling wiki for its categories, Wikidata again for those categories, DeepL for the names), so a long list takes a while. At most 20 articles and 8 other languages per article are examined per run.