Thanx for your answer.
Unluckly, I tried and it did not work even with US langage...
but luckly, I just managed to understand my mistake.
The installation process page always stopped saying there was a problem with friendly urls, but without clear failure message I just believed it was a small warning.... but I just added "AllowOverride All" to my apache configuration file and that's it ! And now I understand that the full installation process page is redirecting to the CMS frontpage.
What is missing there is a clear message saying that the installation FAILED because of the friendly url FAILURE. Because there was no clear message, I was always trying the CMS url and it seems OK... except the TinyMCE error.
My two cents : Do not enable the frontpage until the installation is fully successful and/or make a more clear FAILURE message about the friendly urls checking step.
Thanx for all your work and answers.