Good questions.
Perhaps you downloaded the patch with the wrong language or the patch got corrupted while downloading (redownload it). If this isn't the case you could import the game to GOG Galaxy and let Galaxy update your game (beware that the patch is bigger because it isn't packed - about 7 GB).

== IMPORT ALREADY INSTALLED GAMES TO GALAXY ==

1. For GOG games installed using installers from the last few months (so called Galaxy-compatible installers)
Click the "+" button on top of the sidebar and select "scan and import folders" - it will find all compatible games within that folder and add them to the Client.

2. For remaining GOG game INSTALLATIONS
Find the game in the Library (click on the image of the game), then click the More button and select "Manage" -> "Import folder" and point the folder selector into the folder where that game is installed.
Please be aware that Client will accept any folder pointed to and will overwrite files inside it (this can for example wipe some mods or damage other apps in case of pointing to a wrong folder) and will update the game to the newest version (current default language English).

Source: http://www.gog.com/forum/general/gog_galaxy_beta_2/post1

ok now i will try, i have redownload the patch 3 times but it's always the same error.
now i will try how u say :D

Hope this will help you.

i have installed and imported the game , the launcher is upgrading the game , how can i know if the game is at the last version and if the dlc are installed?

You should see the version number on the start/title screen in the bottom left. For the DLCs there should be some check-boxes in Galaxy. I don't know where to look because I don't use Galaxy. Perhaps behind the more button.

Edit: Just found this:

Download using the Galaxy Client:

1. First when you look at the game page you will see that there are DLCs on the left. The bottom one is the one you are looking for.
Go ahead and buy it, don't worry no payment is required (mine already says owned because I did this step).
2. Now to activate the DLCs press the 'more' tab next to the play button and go to configure.
After you have entered the configure section go ahead and mark your selected DLCs.
3. Enjoy your free content and don't forget to drop the developers a nice word.
Have fun and may your hunt be successful.

Source: http://www.gog.com/forum/the_witcher_3_wild_hunt/guide_free_dlcs_activasion/post1

it s all installed patch and dlc but the game doesn't start if i press play

Are you getting a error message? Is the game running in the background (task manager)? Have you upgraded your display drivers? Have you upgraded your OS to Windows 10?

Have you tried manually going through the patches in order? >1.06 >1.07>1.08

Not sure they are all compatible unless you do it in order

and uninstal any mods. I believe I once had the command mod installed and on update it no longer worked and caused the game crash on launch. (especially being premodtools mods they either dont work or break the game if they do)

He wasn't able to update manually so he had to use Galaxy as a workaround. You don`t need to patch from 1.06 to 1.07 and so on. Just install the latest patch (it includes all bugfixes).
But you are right about the mod part.

@ rikizoo

Are any mods installed?