> Do you know how to retrieve the key code corresponding to the English
> keyboard layout when the active layout is something else, like
> Russian?
No, but there must be a way, because both GTK+ and KDE know how to do
this. GTK+ also can do this when run under KDE, so I'm pretty sure the
information itself comes from X (xkb), not e.g. from GNOME configuration.
No idea about the other way round, but I guess it also works.
> Or maybe the keyboard driver can be configured to return ASCII
> characters when the Ctrl key is held, even when the layout is
> non-English?
Not sure what you mean here, but certainly it doesn't need to be configured
by the user. Maybe by the application/framework (i.e. KDE, GTK+ or Emacs).
> A workaround is to bind C-ы to the same command as C-s (and similarly
> with other combinations that matter).
This is not feasible because Emacs has hundreds of shortcuts coming from
all sorts of places (the core, major and minor modes, my config and so on).
It's certainly not just about C-s.
By the way, same goes for shortcuts/keybindings even without modifiers.
E.g. I have my own keybinding <f12 s> to switch to *scratch* buffer.
<f12 ы> is, predictably, "undefined". However, if using Emacs' own Russian
input method, it works just like <f12 s>, even though the key on itself
types "ы".
Paul