Haruo, I suppose that it's because when they wrote the French lyrics, they just kept "Istanbul" and "Constantinople" and wrote quite different lyrics. Here they are, I found them on lyricsmania

Istanbul, Istanbul !

Istanbul c'est Constantinople
C'est à Istanbul ou Constantinople
Que je suis allé un jour pour y découvrir le grand amour
Que l'on attend toujours

Istanbul ça n'est plus l'Europe
C'est à Istanbul ou Constantinople
Que je l'ai trouvée un soir qui flânait
Au milieu de la foule d'Istanbul

Le muezzin chantait en haut d'un minaret
Et tout le long du Bosphore
Je faisais déjà des rêves d'or
A Istanbul la vie était belle
C'est à Istanbul, je me le rappelle
Que mon coeur fut pris
Par les sortilèges de l'Asie
Je l'ai suivie dans la foule
Un soir sous le beau ciel d'Istanbul
Istanbul Istanbul Istanbul Istanbul

Istanbul c'est Constantinople
C'est à Istanbul ou Constantinople
Que je me voyais déjà arrivé au paradis d'Allah
Qui vous attend là-bas

Oh Istanbul ça n'est plus l'Europe
C'est à Istanbul ou Constantinople
Que je m'approchais en me faufilant
Au milieu de la foule d'Istanbul

Je n'savais comment traduire mes sentiments
Mais en riant elle me dit :
"Comme vous, j'arrive droit de Paris !"

Oh Istanbul c'est Constantinople
C'est à Istanbul ou Constantinople
Que nous avons pris le train qui nous ramène à Paris
Loin du bruit et de la foule, c'est bon de rêver à Istanbul !

Istanbul ou Constantinople
Le bonheur est là, si bon, à sa porte,
C'est bien inutile d'aller le chercher
Je ne sais où Istanbul ou Tombouctou
L'amour il est là tout près de vous

Istanbul, Istanbul !

literal translation (I already said in other threads that I never was a poet nor was I a native English speaker)

Istanbul, Istanbul !

Istanbul is Constantinople,
It's in Istanbul or Constantinople
That I went to discover the love of my life*
The one we're always waiting for.

Istanbul is no more Europe
It's in Istanbul or Constantinople
That I found her strolling
In the middle of Istanbul crowd

The muezzin was singing from the top of a minaret
And along the Bosphorus
I was already having golden dreams.
In Istanbul, life was beautiful.
It's in Istanbul, I remember it,
That my heart was taken
By Asia's spells.
I followed her in the crowd
One evening, under Istanbul's beautiful sky
Istanbul Istanbul Istanbul Istanbul

Istanbul is Constantinople,
It's in Istanbul or Constantinople
That I was already seeing myself in Allah's paradise
That's waiting for you there.

Istanbul is no more Europe
It's in Istanbul or Constantinople
That I came near worming my way
Among Istanbul's crowd.

I didn't know how to show my feelings
But she laughed while saying
"Like you, I've come straight from Paris!"

Oh Istanbul is Constantinople
It's in Istanbul or Constantinople
That we took the train taking us back to Paris.
Far from the noise and the crowd, it's good to dream of Istanbul !

Istanbul or Constantinople
Happiness is here, so good, at your door,
It's useless to go to fetch it
I don't know where, in Istanbul or Timbuktu
Love is here, near to you.

Istanbul, Istanbul !

*To mean "the love of one's life" we say "l'amour de sa vie", but also "le grand amour" (the great love) or "l'amour avec un grand A" (Love with a capital L)