Could you post what exactly you did with /etc/X11/xorg.conf.d/90-keyboard? What modifications did you try? And you remembered to restart X (log out and in again) to check that it works?

According to a quick google, something like this is what should be added to enable UK and RU keymaps, and switch between them using Caps Lock:

Option "XkbLayout"  "uk, ru"

Option "XkbOptions" "grp:caps_toggle"

It is also possible to manually swap keymaps using the following commands:

setxkbmap uk

setxkbmap ru

So if nothing else works, you could make your own keybindings for those two commands to quickly switch between the layouts.

[EDIT]
I see that this thread is a dupe... Did you try mfillpots suggestion lxpanel-xkb from your previous thread?