I use greek locale el_GR.UTF8 and english en_US.UTF8. I have set US English International and Greek Polytonic keyboard layouts. When I use the greek keyboard, dead keys work as expected (eg άὰἇ etc).
However, when I use the american keyboard, they don't: When I type 'a I get a Instead of á, etc. The compose key also doesn't work, which I assume is related. Dead keys work finely as expected under GTK applications.
Any help is welcome, and please ask any information necessary.