The other spellings come from:
1) original working title (working titles don't stick)
2) Non-English countries.
http://www.imdb.com/title/tt0118887/releaseinfo
North American distribution uses "Cop Land" over other variants.
you might not be from North America, but I doubt there's any internation versions of the OST that differ from the NA releases.
USA/Canada share the same distribution.
In rare cases, a different country may add a bonus track or contain different material, but not that I know of for Cop Land.