According to How to use Alt + GraveAccent on non-US keyboard?, the Unity shortcut to switch between windows of the same application is Alt + (whichever key is above Tab).
It doesn't work in my DELL-SK8115 keyboard with Brazilian ABNT2 layout. It's like the key is broken (i.e. pressing the combination has the same effect of pressing only Alt).
Also, if it helps, the same key (but in the Ctrl + ` combination) doesn't work in Sublime Text 2, either.