The Date display format is controlled by the Locale settings.
Even after setting the locale, we were facing issue with the display format of date in non-ENU language. With the same user id, it was showing different date display formats,
Furthur investigations revealed that it was because of the Languge settings in the Server configuration that was creating the problem. Following changes were done to fix this.
Navigate to Sitemap>> Server Configuration >> Enterprises >> Component Definations.
Search for Component Parameter name = Language Code, Make sure that the OM language matches with the language name here. (Eg : if the OM is for PTB, the language code here should be PTB).
Syncronize after the changes, and bounce the Siebel servers.