Default date, time, and language settings for the whole organization
The administrator can set up the default date, time, and language for the whole company in Configuration → Account settings:

The administrator can change the date, time, and language settings for an employee
If you (as an Administrator) want to change the date and time format or language for one employee:
Open Configuration → People.
Double-click on the employee for whom you want to change the settings.
Click the Settings tab.
Check Activate custom settings, make the changes and click Save.

Each employee can change their personal settings too
Each employee can choose his own date and time format and language interface
Open the menu in the bottom left corner of the screen and click User Profile.
Enable Custom settings and make the changes.
Click Save
