Isn't AltGr the usual key to get alternate characters instead of Alt which is for keyboard shortcuts? I've never seen a program mapping shortcuts to AltGr.
"ś" is input in Polish using AltGr+s (or Ctrl+Alt+s), but AltGr is not labeled as such on Polish keyboards (it's simply the right Alt key, and people will refer to it as such when asked).
Most North American keyboards don't have AltGr, they just have 2x Alt keys.