Each language file is annotoated to tell you what items are new and changed. For example, the calendar language file...
QUOTE
// 2.1 (NEW)
'cal_event_delete' => 'Calendar event removed',
'button_delete' => "Delete",
'button_edit' => "Edit",
'button_approve' => "Approve (Make Visible)",
'button_unapprove' => "Un-approve (Make Invisible)",
'new_event_mod' => "Your event has been posted to the moderation queue and will require approval before it's shown",
'day_add_event' => 'Add New Event',
'show_n_events' => '(%s) Events',
'form_whichcal' => 'Add to Calendar',
'form_tz' => 'Event Time Zone',
'calendar_group_filter' => "<b>Admin Option</b><br />(only applies to public events)",
'calendar_delete_box' => "Delete Event",
'calendar_title' => "Event Title",
'calendar_event_date' => "Date for Event",
'calendar_event_type' => "Type of Event",
'calendar_type_private' => "Personal Event (Viewable by you only)",
'calendar_type_public' => "Public Event (Viewable by all)",
'calendar_submit' => "Add Calendar Event",
'calendar_edit_submit' => "Edit Event",
// 2.1 (CHANGED)
'entry_birthdays' => "(%s) Birthdays",
'post_new_event' => "Manage Calendar Event",
//------------------------------------------------