Thanks for checking that. My concern is that there could be a caching issue related to the set locale from accept language header setting that would cause the 404 page to be displayed with the incorrect locale under some circumstances. It seems likely that is what happened the previous time that you accessed that page.
There were some recent changes made to how Discourse caches pages for anonymous users. Those changes should prevent this from happening, but it’s possible that there are still issues with caching the 404 page.