OpenMPT works with virtual scan codes and not hardware scan codes, and I suspect that changing your layout to cyrillic mode will result in OpenMPT receiving completely different scan codes. I guess I can try responding to WM_INPUTLANGCHANGEREQUEST messages and refresh the keyboard layout whenever such a request is sent.

Alright, this boils down to OpenMPT still receiving the same scan codes, but they translate into different (cyrillic) characters if the current key map says so.
I've modified the code so that it also scans the second installed keyboard layout for effect letters A-Z if it can't find them in the current layout. It seems to work for me when adding a Russian keyboard layout here, so please check if it works as intended for you: http://sagagames.de/stuff/mptrack.exe

You will still have to focus a different song window or a non-OpenMPT window after switching, though, as the WM_INPUTLANGCHANGEREQUEST layout change notifications seem to be kinda broken since Windows XP and the replacement stuff is hardly documented.

Yeah, it works correctly for me now! Thank you very much, Saga Musix!!!

Hey Fler0301, are you still around? I introduced some changes to how OpenMPT treats some parts of its keyboard layout and I need someone with Cyrillic+Latin keyboard maps to verify that it works as intended.
Preferrably, answer in this thread here to keep things centralized: https://forum.openmpt.org/index.php?topic=3286.msg43751#msg43751