Why 'lef' in lieu of 'lieu'?

[Rolferoo]

Why do British and Commonwealth security forces pronounce the rank 'Lieutenant' as 'leftenant'?

[Quizmonster]

The word 'lieutenant' means '(one) holding the place of (another)' from the French root-words. However, in Old French, both 'lieu' and 'luef' were used to mean 'place'. English seems to have adopted the apparent sound of the OLD spelling, whereas Americans etc chose the more MODERN.

[Pinotage]

Further to quizmaster, I recall hearing that the lieutenant was the officer responsible for the left flank of an army. I missed the explanation of why there isn't a rightenant