Different editions of LOTR?

In an futile attempt to improve my long forgotten French I´ve tried to read a French pocket edition of LOTR along with my old English edition from 1962. I was surprised to find some differences, indicating that the French book must have been translated from a slightly different version of the book.
For example, in the chapter "Many partings", just after the encounter with Saruman, my version goes like this [img]smilies/frown.gif[/img]Gandalf):
"-All the same, I am not sure that Treebeard is right: I fancy he could do some mischief still in a small mean way."
And then, in the French version follows this:
"Le lendemain, ils poursuivrent leur route dans le nord du Pays de Dun, où ne demeuraient plus d´hommes, bien que ce fût une région verdoyante et agréable. Septembre venait avec ses jours dorés et ses nuits argentées, et ils chevauchèrent tranquillement jusqu´au moment où ils atteignirent la Rivière des Cygnes; ils trouvèrent alors l´ancien Gué, à l´est des chutes par lesquelles elle descendait dans une brume les marais et les ilôts parmi lesquels elles serpentait jusqu´au Flot Gris : lá d´innombrables cygnes gitaient parmi les roseaux.
Ils passèrent ainsi en Eregion et enfin vint une belle aurore, qui rayonnait au-dessus de brumes chatôyantes; et, regardant de leur camp sur une colline peu élevée, les voyageurs virent le Soleil dans l´Est lointain sur trois cimes qui se dressaient dans le ciel à travers les nuages flottants : le Caradhras,......"
In my English version Gandalf's words are followed by only this:
"September came in with golden days and silver nights. At last a fair morningdawned, shimmering above gleaming mists; and looking from a their camp on a low hill the travellers saw away in the east the Sun catching three peaks that thrust up into the sky throughfloating clouds: Caradhras....."
I wonder if anyone could tell where the French version comes from. Is it from the original edition, or something added in a later edition than mine? As far as I remember I haven't read about the "Swan River" anywhere else, or seen the name on any map. (But evidently it's the name of the tributary to Greyflood that comes from the gate of Moria). I hope I'm excused for not trying to translate the French text back into Tolkien's English. As You can see it's not even my native language, but I doubt I would have dared to do so anyway.