Integration with the Add-ons Manager, the toolbar button and all the available options should work as expected.
Our "User Interface Language" dialog isn't accessible from the main SeaMonkey preferences window, as a similar alternative is implemented by the application (Preferences » Appearance » User Interface Language).
The toolbar button is available for the "browser" and "mail" components. No plans for to made it available for the rest.
More appropiate links for to get language packs in Nightly and Aurora builds are pending until the fixing of bug 902876.
Version 0.8 adds compatibility with SeaMonkey: