Tested on:

1. Drupal 6.16+
2. Date API 6.x-2.4
3. Internationalization 6.x-1.3

Any date format is stored as system variable (on the global $conf variable).

Since Internationalization module allows to declare some system variables as Multilingual, you could add to your $conf[‘i18n_variables’] on settings.php these lines to use different date format for different languages:

$conf['i18n_variables'] = array(
// Other variables
// bla bla bla
// Date variables
'date_format_long',
'date_format_medium',
'date_format_short',
'date_first_day',
);

date_format variables are Long, Medium and Short date format, used in many places (including Views).

date_first_day is the first day displayed on calendars (e.g. Sunday for English, Monday for Italian).

Note that you have to save the value twice via:

http://example.com/it/admin/settings/date-time
http://example.com/en/admin/settings/date-time

And one more time:

http://example.com/it/admin/settings/date-time

After the first time, you can change format as you like without double checking.

See also:

• Multilingual variables (Internationalization) howto