Locate the game installation path,following:

DefEd\Data\Localization\language.lsx

back it up and open it with proper text editor, change:
<attribute id="Value" value="English" type="20" /> to <attribute id="Value" value="French" type="20" />

It should work.

Dear J1mZY_76,

Thank you for your answer... but no success with this change. I saw that in some forum and this is the first thing I did, but it didn't make any difference.

I think that, at some point, I took out of the folder The file English.pak, the file french.pak and the file language.lsx... and the game was running undisturbed in English.

I send a mail to GOG and I am waiting for some more ideas

In installer You have language options. On first screen.

BTW: it shoud be more visible.

Hi p0k3m0n,

Thanks for your comment. The point is that my game was installed with GOG Galaxy a long time ago. Only recently I added the installation of the Definitive Edition over the Classic Edition. So, I didn't pay much attention, at the time, about the language.

But whatever the language of installation it is just an original mistake from the very beginning.

Somehow the game itself is done in english. When we speak about the different languages we speak about the "subtitles" part of the game.

So it is obvious that, in the menu "Options/Games" when they ask to Enable Subtitles a sub-menu should have been available to get the language of the subtitles...

But the game is gorgeous... even with english subtitles :op

Dear friends,

I received a very exhaustive (and for me who is new to the PC Windows world... quite overwhelming) answer from DrCAT at GOG.com.

To my great shame, the obvious and easiest solution was the one.

I said before that I installed the game "Original Sin II Definitive Edition" via the program "GOG Galaxy".

So for me, the solution was to click the "MORE" button to choose the "Settings" option.

And there it was...the menu to choose from this darn french language :o)

Thousand excuses for my stupidity of not finding this so obvious solution by myself.

Still, it was nice talking to you :o)

Have a blissful day