Among the many reasons that make it so difficult to translate a song between French and English, there is of course the inevitable mismatch in the way that words rhyme and have various connotations. But with some imagination and re-creation, you can usually cope with that. However, what you can't escape is the difference in information density per syllable between French and English.
French just requires more syllables to say the same things. You may have experienced this phenomenon when visiting bilingual blogs. I did when translating my own writings and other texts: the English version is always noticeably shorter (about 10%). Well, when you try to translate a song, this effect matters a lot, because you have to fit the translated text in the same melody (though some melodies allow for a bit of cheating). You can make do when there are few rhymes; when there are many syllables between rhymes; when the rhymes and syllable count are lax. But when there are many rhymes with few syllables in-between, you're completely stuck. And of course, it only gets worse since you have to respect which syllable of which word may be stressed by the melody. Ouch.
Here is a translation I did a few months ago in a relatively
It is kind of ok, but you can easily see that something's missing, notably many rhymes.
And no, I won't reveal who was the X in the end of the song
for whom I did this.
Poets often use many words
Fly me to the moon
Fill my heart with song
Les poètes s'étendent longuement
Emmène-moi sur la lune
Emplis mon coeur d'un chant
OK, so this particular song was translatable, as least badly. I can show you many songs that aren't at all (take What Kind of Fool Am I as sung by Sammy Davis, Jr.). In these cases, all you can do is create a completely different song with the same melody: Johnny Mercer did The Falling Leaves with Les Feuilles Mortes; and then there is My Way and Comme d'habitude. Very different songs, with the American versions having much more positive view of life than the French, the one having fighting spirit where the other one is acceptation of sorrow. That's a reason why I find myself mostly unable to appreciate love songs in French; although I suspect another reason is that my critical mind is less developed in English, not having drawn the emotions that would tell me how silly the words seem in English. In any case, the problem of translation is not just lexical, it's about all the mismatch between two cultures.
NB: If you like translation, a must read book is Douglas Hofstadter's Le ton beau de Marot.
PS: Information density per syllable is all the more important since songs always try to pack as much emotion as possible in each group of verses.