The "problem" is that the servers around the world is not configured the same way, and it's also different between windows and nix servers.
When putting in all known country locals in the language file the server has something to "chose" from, like in your case the server is not set up with country local sv_SE.ISO_8859-1 so it's choosing the one it knows - sv_SE
Hope this made it clearer. :)
PS! I see that the Swedish translation team for Joomla is using the following country locals for the upcoming Joomla 1.5:
sv_SE, sv_SE.ISO8859-1, sv_SE.ISO8859-15, sv_SE.iso88591, sv_SE.iso885915, swedish, sv_FI, sv_FI@euro, sv_FI.iso88591, sv_FI.iso885915@euro, swedish_finland