Summary
By default, BSM 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 onwards, it is actually possible to manually change the date format of a supported locale or to add support for an additional locale yourself. However this is not officially supported, but the attached PDF describes what needs to be done in case you want to try it out anyhow.
Note that a BSM patch may override the described changes and you might therefore have to re-apply them after a patch installation.