With v0.155.0 and later you cannot create an alias to the server root using the aliases feature. Alias paths starting with a slash (/) resolve to the site root, not to the server root.
Perhaps there’s another way to handle your use case; I’ll give it some thought.
This is the setup and objective, as I understand your use case.
baseURL = 'https://example.org/'
defaultContentLanguage = 'en'
defaultContentLanguageInSubdir = true
[languages.en]
weight = 1
title = "My Site in English"
[languages.de]
weight = 2
title = "Meine Seite auf Deutsch"
content
content/
└── s1/
└── p1.en.md
desired result
public/
├── alias-to-p1-en/
│ └── index.html
├── de/
│ └── index.html
├── en/
│ ├── s1/
│ │ ├── p1/
│ │ │ └── index.html
│ │ └── index.html
│ └── index.html
└── index.html