[O] Timestamp language

[Fredrik]

When I add a new timestamp by either pressing C-c . or C-c ! the 
calendar shows up in english but when textrepresentation of the date the 
dayname is shown in swedish.

So adding a date for a wednesday would give me <2013-08-14 on>
I just can't find how to change my language settings so I get this in 
english instead. Any ideas?

Regards,

Fredrik