This is an old question about Ubuntu in 2008, but if anyone wants to know how to do this in 2017 with Ubuntu 16.06, it is described here. Cut/Paste follows below in case that site goes away:
Change date and measurement formats
You can control the formats that are used for dates, times, numbers, currency, and measurement to match the local customs of your region.
- Click the icon at the very right of the menu bar and select System
- Open Language Support and select the Regional Formats tab.
- Select the region that most closely matches the formats you'd like to
use. By default, the list only shows regions that use the language
set on the Language tab.
- You have to log out and back in for these changes to take effect.
Click the icon at the very right of the menu bar and select Log Out
to log out.
- After you've selected a region, the area below the list shows various
examples of how dates and other values are shown. Although not shown
in the examples, your region also controls the starting day of the
week in calendars.