Apple script to lock screen using keychain's menu item

gistfile1.txt

(*
Probe Menu Bar

This script uses UI element scripting to return a list of every menuitem
of every menu for every application currently running.

If "Enable access for assistive devices" is not checked,
this script will open the Universal Access System Preference and ask
the user to check the checkbox.

Copyright © 2007 Apple Inc.

You may incorporate this Apple sample code into your program(s) without
restriction.  This Apple sample code has been provided "AS IS" and the
responsibility for its operation is yours.  You are not permitted to
redistribute this Apple sample code as "Apple sample code" after having
made changes.  If you're going to redistribute the code, we require
that you make it clear that the code was descended from Apple sample
code, but that you've made changes.
*)

tell application "System Events"
	get properties
	get every process
	if UI elements enabled then
		tell process "SystemUIServer"
			repeat with i from 1 to number of menu bar items of menu bar 1
				if description of menu bar item i of menu bar 1 is "Keychain menu extra" then
					tell menu bar item i of menu bar 1
						click
						if name of menu item 1 of front menu is "Lock Screen" then
							click menu item "Lock Screen" of front menu
							exit repeat
						end if
					end tell
				end if
			end repeat
		end tell
	else
		tell application "System Preferences"
			activate
			set current pane to pane "com.apple.preference.universalaccess"
			display dialog "UI element scripting is not enabled. Check \"Enable access for assistive devices\""
		end tell
	end if
end tell

The strings "Keychain menu extra" and "Lock Screen" are localized strings. So you have to edit them to fit to the language your system is running.

In German these strings are "Schlüsselbund-Menüerweiterung" and "Bildschirm schützen".