It is extraordinarily difficult. But it can be done. For example, Alasdair Clayre has done it - here is The Dove, his translation of Brel's La Colombe.

And Jake Thackray has done it with some Georges Brassens songs.

But it can't be done by some hack who is just trying to write a song for some show-biz celebrity who heard the tune in a bar somewhere. And there's got to be sensitivity to the underlying meaning, and the nuances of language.

For example - and I may be wrong here "Quand on n'a que l'Amour" doesn't mean "If we only have love" but if "If we have only love" (ir strictly "When someone has only love") which to me means something subtly and significantly different.