Exchange 2007 OWA provides a new screen to select your language and time zone when you login for the first time.
In order to set the default language for OWA, run the following command.
Set-OWAVirtualDirectory “owa (Default Web Site)” -DefaultClientLanguage LocalID.
Local ID is a decimal number given for each language. Check your language’s equivalent local id here
In my case, Local ID for English -United Kingdom is 2057. So, the command will be
Set-OWAVirtualDirectory “owa (Default Web Site)” -DefaultClientLanguage 2057.
Though you can set the default language, users can always change the language by going into Options -> Regional Language in OWA 2007.