Redirecting to multilang URL after language refactoring

Hi,
I am refactoring the multilingual structure for the website and moving every page that was under / to /de and everything that was under /en to /. To avoid complete breakdown (a lot of those links are ranked pretty high), I was trying to find the solution with redirects. Surely, I’ve read the URL Management | Hugo but I still can’t find how to set the alias/redirect for those pages correctly. Here is an example of the _index.de.md which I have used before:

---
title: "Page title"
date: 2017-11-21T21:28:43+01:00
description: "Some meta description."
keywords: "some keywords"
draft: false
layout: single
type: industry-a
url: branche-a
aliases:
- industry-a
---

So the old URL was domain.tld/branche-a which now needs to redirect to domain.tld/de/branche-a
Moreover, the old domain.tld/en/some-url needs to redirect to domain.tld/some-url

Setting another alias in the frontmatter example above like this:

url: branche-a
aliases:
- industry-a
- de/branche-a

didn’t help. Any ideas?

Sometimes writing down a question and then trying harder gets your the result:

url: branche-a
aliases:
- industry-a
- ../branche-a

The path relative to root did the trick.