It seems that the not-found page treats users as anonymous users. We have the set locale from accept language header setting enabled on Meta. This setting is intended to set the locale for anonymous users to the locale that’s sent with their browser’s language headers.
When I set my browser’s locale to French, the not-found page uses the French locale even if I’m logged into Meta. This might be acceptable, but if a locale that is not set in your browser’s language settings is being displayed for you, it is a problem.
Are you able to check your browser’s language settings? On Chrome, the language is set in the Advanced settings:
