Hi.

I post this because i had both problems xD.

Blackscreen: Looking in san-Google i read something about a problem with Directx. Here my solution (works for me :D)
1.- Find Assassin's Creed folder
1.a.- right click in your .exe
1.b.- Properties
1.c.- Open location
2.- Find AssassinsCreed_Dx9 and execute
3.- Enjoy (blackscreen solved for me)

Language change: I downloaded the game in spanish but when i start the game was in english. Here my solution
1.- Start (windows icon)
2.- All programs
3.- Find GOG.COM folder and open
4.- Open Assassin's Creed folder
5.- Open Change Game_Language
6.- Choose your language and start the game in that windows
7.- Enjoy

Hope this helps and sry for my bad english