Then for each site collection go to settings -> Site Administration -> Regional Settings.
Then from the drop down select the Locale for the site and tick the check box labeled "All sub-sites inherit these Regional Settings."
This will configure date formats, language, currency and other formatting issues to the appropriate regional setting.
The tricky bit is the setting for the My Sites. Each my site is self-contained in its own site collection therefore it will have it's own regional setting defaulted to US Format. In order to deal with this issue you will either need to manually visit each my site after creation and do the same thing in the UI which is ok for organisations of 10-20 people. But in large organisations it is recommended you implement a feature stabler onto the site definition for SPSPERS#0, the feature can then set the locale during the feature activation event via .NET code.