The reply from Jeganarayanan may be fine for a Windows PC but the instructions in the linked page can't be followed on a Mac even though the principle is similar.
On a Mac you need to go into > System Preferences - Language & Region to change the setting then Restart your Mac. When you launch any of the Mac Office programs the menus, dialogs, dates. times. currencies, etc. will be displayed according to the new settings.
NOTE that this is a system-wide modification to the Mac, so all software as well as the OS, itself, will be affected by the change.