The number:format-source attribute specifies the source of definitions of the short and long display formats. Unfortuantely this seems to be restricted to the interpretation of what long and short means with respect to date and time components, ie. what are short minutes or long minutes.
Users on the other hand may want to specify that a "long date format" is to be used or a "short date format" and expect that the order of the components is adjusted for the current locale. Prorams such as GNumeric procide this ability to the user. IN fact the default formats used have that character. Unfortunately currently it appears to be impossible to export such a format.