One of the many beautiful and deceptively simple poems of Prévert. There are a few cleverly used idioms.
clair comme le jour -> (as) clear as day (very easy to understand)
simple comme bonjour -> (as) simple as ABC / easy as pie / a piece of cake (very easy/simple, can be about understanding or doing something)
nu comme la main -> naked as a jaybird (or possibly as a needle) (completely/stark naked)
"naked as a jaybird" would be pretty nice for the story (the bird flying out the window)
The 2nd stanza sounds like their love is a kind of cute pet, "l'amour" is like "the cat" or "the dog". The idea is, if the little puppy gets overexcited it may want to run away :)
What our life would be -> "what would be our life" or "imagine/picture (what) our life (would be)"
valse -> waltz
la mécanique de l'ennui, sans amour, sans vie -> something like "'a clockwork boredom, devoid of love and life"
arc-en-ciel
The translation is done by Bahareh Tajfirouz.Please ask for permission if you would like to use or reprint it.