Unicode and modifier keys

I have recently added Unicode support to a GUI library I maintain.
The Library is called GG, and it works with SDL
(http://sourceforge.net/projects/gigi for the curious).

I am a US-keyboard and English-only user. I am trying to determine
what is normal for Unicode input, and I have no way to test Unicode
support to see for myself.

It seems from the code in src/video/x11/SDL_x11events.c that it is
impossible to get a value for keysym.mod other than KMOD_NONE when
keysym.unicode is nonzero. Again, I have no way of verifying this
other than by inspection of the code.

I have a Polish user who seems to be saying that he always sees
keysym.mod=KMOD_MODE when processing Unicode key events. I’m not sure
this is accurate, simply due to the language barrier between us. Does
this sound reasonable?

So, here are my 2 questions:

  1. Is my reading of the X11 code correct, and in general what mod keys
    might I find when keysym.unicode is nonzero, not just for X11 systems,
    but on all platforms?

  2. SDL aside, what should I expect from non-ASCII keyboards when
    Unicode key events are generated – are there certain mod keys that
    are commonly used to generate non-ASCII characters?

Zach Laine