It's fine if Jo Stafford records "La Vie en Rose" in English. It's not clear why Jo Stafford has to record "La Vie en Rose" at all, but if she records it, she may as well do it in English.
But Edith Piaf should not. Her songs are immeasurably better in French, as the translations are invariably rather insipid, and she shouldn't pander. Let's just compare, from the two versions of the great "Rien de Rien":
"No! No regrets.
No! I will have no regrets.
For the grief doesn't last, it is gone,
I've forgotten the past."
"Non, rien de rien!
Non, je ne regrette rien!
C'est payé, balayé, oublié--
Je me fous du passé!"
"No regrets" has all the oomph of a sick kitten when compared to "Rien de rien." It loses, among other things, the visceral emotion caught in the French rhotics. It also loses the superb, rhyming ascending tricolon of payé - balayé - oublié, and the passion of Je me fous du passé, for which "I've forgotten the past" is a lifeless substitution.
I have regrets.