I pronounce it Leftenant, as it should be pronounced. If my character speaks with an accent, I spell it that way too.
Actually, the pronunciation of
lefttenant is strictly when referring to a commissioned officer in the British and Canadian navies ranking just below a lieutenant commander.
Lieutenant - (with the oo sound) - is the correct pronunciation when referring to a commissioned rank in the U.S. Navy or Coast Guard, an officer in a police or fire department ranking below a captain.