As long as you have not used Emacs for switching layouts was Alt+Shift.
But Emacs is the combination required.
To put in Ctrl+Shift too, Nelya, because in Ubuntu this combination is often used (Nautilus for example).
In the end I use CapsLock, but I don't like it.
Tried Alt+Space (schematically as on the Mac), but not too much.
Therefore, I ask Council, what keys to put to be comfortable and not interfere with Emacs'?