Accented characters composition

I use US keyboard with deadkeys. When writing foreign language text I use compose keys. For instance French: "Alt" + "," + "c" => i This works system-wide, in Console, KWrite, Kate, OpenOffice and Firefox. However the Hungarian special characters: "Alt" + "=" + "o" => and "Alt" + "=" + "u" => work only in OpenOffice and Firefox (as you see here :-)). It does not work for Console (no problem, little use for accented characters in Console), but neither for KWrite and Kate (this is where I want to use them). Any idea, how I could generate them? (And why they are OK for OpenOffice and Firefox but not for the others?)