I read in an old post somewhere (which I can no longer find on google) mentioning if you manually set the value of WEB3LOCALE cookie to "uk" (rather than "en"), then you can get European date format PLUS English user interface both at the same time.
The code for this "en" or "uk" setting are here if you wish to recompile to hard-fix it at "uk" mode:
dcm4chee-web-common/src/main/java/org/dcm4chee/web/common/login/LoginResources.java line 16
dcm4chee-web-common/src/main/java/org/dcm4chee/web/common/base/ModuleSelectorPanel.java line 289