The Problem
Collabora Online simply refused to display the German interface when accessed from SharePoint SE as a WOPI host (Windows Server 2022). In our setup, Collabora Online runs on a Docker container with a reverse proxy (we had tested Traefik and Apache, as well as Collabora Code and the full version). SharePoint was set to German (LCID 1031); IIS also had the HTTP_ACCEPT_LANGUAGE header set to German; the browser language was also German.
In the developer tools, we also saw that the localization is loaded depending on the browser language—so the correct JSON was reloaded but not applied.
The Solution
After a lot of trial and error, we finally found a solution. It really comes down to the call to cool.html. Unfortunately, a URL rewrite rule on IIS doesn’t help here. It’s important that the language parameter is set correctly. Calling cool.html?lang=de&
This is exactly where the devil is in the details—with the reverse proxy, the & character must be escaped, for example cool.html?lang=de& —that was the solution for us. Now every call is in the desired language, German.
You may need to deleted the old WOPI bindings and set the bindings new in SharePoint for this to work.
Hope this solution helps you! And thanks again to everyone who attended the Cool Days in Hamburg 2026—it was a great exchange.
Best regards,
Sven