Hello @eaccelera Welcome to collabora online forum 
In Docker-based setups of Collabora Online (CODE), the UI language is primarily determined by the browser language and client request headers, not directly by the extra_params or the coolwsd.xml settings in most cases.
1. Check Browser Language Headers
Ensure the client (browser accessing the document) sends the correct language:
- In Chrome or Firefox:
- Go to browser settings → Languages → Move Deutsch (German) to the top.
2. Set lang in WOPI host request (if possible)
If you’re controlling or customizing the WOPI host (e.g. Seafile integration), the WOPI URL used to open the file can include a lang=de parameter. This helps force the UI language.
For example:
https://<your-collabora-domain>/loleaflet/2.0.13/loleaflet.html?WOPISrc=...&lang=de
However, not all WOPI hosts support adding arbitrary parameters like
lang. Seafile may not expose this easily.
I found this discussion in old post might be interesting to read : Collabora CODE (with seafile) default language