I use openSuse 11.2, but I assume that it's the same.
Anyway, I use a Swedish keyboard layout and have English as system language. It works perfectly for me. During the installation the keyboard layout and the system language were two separate choices, that's where I set it.
I took a look in yast trying to find where to set the default settings, but as you said I only saw one language setting, and that was the primary language (system language I guess).
However, I found a user-specific setting for the keyboard layout. In Configure Desktop (find it in the menu or start it with the command "systemsettings"), click on the "Regional&Language" button, click on "Keyboard Layout". Here you can change the keyboard layout, but it has to be done for all users separately.
There really should be a way to set keyboard layout that isn't user-specific, but I cannot find any at the moment.