why do the english say leftenant
LEFTenant is the way British troops pronounce the rank Lieutenant. The two words mean different things within different military groups, but there is no actual difference. A British troop can refer to an American Lieutenant as  leftenant and not be incorrect.
It's only the difference in pronunciation. In English, lootenant is chiefly the American pronunication and leftenant the British. In the British and U.S. armies, 1st and 2nd lieutenants are junior officers, and the ranking system is roughly equivalent between the two armies.