Just to make this more complete, this is also happening with the installer for the MATE version based on glibc, it’s a general problem with the installer on this flavour.
It doesn’t matter which keymap you pick in the installer, as soon as you reboot and get to the login screen, you’re scre*** if, for instance, you picked a strong password for your account using symbols, as you are stuck with the US keymap on the login screen and most likely you can’t/don’t know how to type those symbols.
After you log in, the US keymap is still in use until you get to the Keyboard prefs editor or something else to solve the problem.
The funny thing is, on the init screen I can clearly see the message
Setting keymap to "es"
but just like the cake, this is a lie!