Setting DATEFIRST to 6
Can i add another language based on an existing one, and set de field datefirst to 6 ?
Can i modify an existing syslanguage ?

You cannot modify sys.language in 2005 and up.

All what i want, is te set it to 6 parmanently, because i need it to get the right week number.

Is there any specific reason to do this at sql server level ? Why cant this be done at the application level ?

http://www.sqlteam.com/forums/topic.asp?TOPIC_ID=62891 has a method to do it using registry changes, but be CAUTIOUS of doing it without proper testing !!

Edit: Added my comment in answer:

How about using SET DATEFIRST N (where N = 1 through 7 and 7 is default). Ref: msdn.microsoft.com/en-us/library/aa259210(v=sql.80).aspx