I try to change the language with the keyboard as there is no mouse support from the boot menu. The keyboard doesn't respond! My keyboard is an USB, so I suppose Ubuntu's boot manager can't handle USB keyboard?..
Depending on your hardware, you may need to enable Legacy USB support in the BIOS.
(Not just a Linux problem -- I've also seen Windows being unable to use the keyboard to select 'safe mode' after hitting the F8 on bootup without that feature being enabled)