I've done a new Exchange 2010 SP1 install today and was told that the bug in RTM where setting the OWA default language broke OWA and ECP was fixed in SP1. Is it heck. I used the cmdlets: Set-OwaVirtualDirectory -identity "Owa (Default Web Site)" -LogonAndErrorLanguage 2057 and: Set-OwaVirtualDirectory -identity "Owa (Default Web Site)" -DefaultClientLanguage 2057 and, guess what? OWA broke. Same old chestnut - go to open OWA options and it errors. try to use ECP and it wants to set language and then errors.
Thankfully I still had a record of the workwaround - unset the language by running the same cmdlets but choosing 0 as the value instead of 2057. Come on guys, this has been broke for 10 months now....