This document has not been formally reviewed for accuracy and is provided "as is" for your convenience.
Summary
OMi takes the locale of the browser in order to select the date format, however not all locales are supported. In case a browser locale is not supported the fallback is to en_US.
From BSM 9.23 on it is actually possible to manually change the date format of a supported locale or to add support for an additional locale yourself. However there are some things to consider and this is not officially supported. See the attached a PDF that describes what needs to be done.
Note that a BSM patch may override the described changes and it is required to re-apply them after a patch installation.