Thanks @maiki, this works. But can you explain why? Is slug only used for single pages and url for list pages, is that the explanation / reason slug didn’t work?
My example was simplified. I wanted to rename the permalinks for translations: url: categories → _index.en.md (guess I could omit this because it’s default) url: kategorien → _index.de.md
and so on.
I’m trying to figure how I knew that, and I figure it was gleaned from experience. Permalinks are patterns for normal pages, not lists, which a section root is. As you wanted to override a list page via front matter, it wouldn’t otherwise pick up a permalink pattern.
In the future, I’d be verbose with the live examples. Had you mentioned your area of thinking you would have gotten numerous responses from multilingual users having solved your exact issue.