wish: right alt/meta to switch keyboard layout while pressed

[Paul Pogonyshev]

Hi,

It is probably a bit too late now for wishes, but maybe it is trivial
to implement in Emacs, so I'll ask just in case.

There is a useful feature to switch keyboard layout temporary while
the right alt (meta) key is pressed.  In xkb you can turn it on with
`-option grp:switch'.  It is useful, because some characters are
absent from some non-English layouts and `normal' switching back and
forth is quite annoying for a mere character or two.  For instance,
you cannot type `[', `]', `{', `}', the apostrophe and some other
characters while using Russian layout.

So, if I want to type an apostrophe from Russian keyboard layout, I
have to type `C-\ ' C-\'.  If, however, the temporary-switching was
implemented, I could do with `press-right-alt ' release-right-alt',
which is obviously much easier.

As far as I can tell, Emacs currently just ignores the right Alt.

Can this be implemented easily?

Paul