... and some other
I tried also 'systeminfo', or registry ("HKEY_CURRENT_USER\Control Panel\Desktop\MuiCached"), or even wmic and so on... All of them responded mostly with system locales or which language OS was installed originally instead of telling which MUI is used.
This one finally help:
reg query "HKEY_CURRENT_USER\Control Panel\Desktop" -v PreferredUILanguages
If you need this to store as variable just use FOR command with /F parameter:
FOR /F "skip=1 tokens=3" %%I IN ('reg query "HKEY_CURRENT_USER\Control Panel\Desktop" /v "PreferredUILanguages"') do SET Current_MUI=%%I
Basic info about MUI: