legluondunet: Hello,
I'm surprise GOG does sell SP1 in English and German only while the game is released in more languages.
This archive contains all 5 languages english, french, italian, german and spanish Tom-s Clancy's Splinter Cell files:
https://www.dropbox.com/s/xjdqhzvbp05hjce/sc1_multi5.zip

Extract the archive in main game folder, edit "SplinterCell.ini" config file located in "Splinter Cell/system" folder and modify the language option in [Engine.Engine] section like this:

english:

[Engine.Engine]
Language=int

or french:
[Engine.Engine]
Language=fra

or italian:
[Engine.Engine]
Language=ita

or german:
[Engine.Engine]
Language=deu

or spanish:
[Engine.Engine]
Language=esp

Have good game!