(xah-fly-keys-set-layout "russian") can't use english system layout with the leader key

[megumintyan]

It works good with RU/US system layouts in command mode but the leader key prints "undefined key" if I use US system layout.
Also if set "qwerty" layout after "russian"

  (progn
    (xah-fly-keys-set-layout "russian")
    (xah-fly-keys-set-layout "qwerty"))

then the RU layout still works in command mode, but the leader key only works with the US layout.
Is it a bug?
In addition, this behavior suits me if it is impossible to make both layouts work simultaneously with the leader key.