The problem has the same symptoms as the issues mentioned in this thread:

[SOLVED] Errors in beta 4 Spanish version

except this time it's in the Swedish version. It's broken in both beta4 and beta5 releases (as of build 1433)

Here are the steps to manually fix it:

1) Quit Zimbra Desktop app:

On Mac or Linux, just simply close the app window; on Windows, right-click on systray icon->Quit

2) Go to "xul chrome" directory:

On Mac, it's "macos/prism/>$ linux/prism/xulrunner/chrome"; On Linux, it's "linux/prism/xulrunner/chrome"; and On Windows, it's "win32/prism/xulrunner/chrome"

3) rename sv.jar to sv-SE.jar

4) Restart Zimbra Desktop