Ingmar,
Google Apps works fine with EE. After reviewing this thread, EH removed conflicting MX records from their DNS servers. Now the email side of things works fine.
However, even after I re-uploaded all language files, both English and Korean, it still displays an error page right after clicking the submit button without any errors shown as follows:
http://screencast.com/t/NmE4N2M4N
And the same error occurs when you trying to activate a test user by clicking the received activation link sent via email.