Man pages matching name slick-greeter-set-keyboard-layout in section 1:

slick-greeter-set-keyboard-layout.1.en	debian 10 / slick-greeter
slick-greeter-set-keyboard-layout.1.en	debian 11 / slick-greeter
slick-greeter-set-keyboard-layout.1.en	debian 12 / slick-greeter