Different language per user in MATE and change default lang of greeter

[john doe <johndoe65534@mail.com>]

Hi list,

My system locale is 'C.UTF-8'.
Obviously, this is not desirable for the users.
So I want to change the language per user in MATE, so user1 would have 
language 'a', user 2 lang 'b' and so on.
Thanks to the list I have all language pack installed for both users 
('a' and 'b').

I have two questions:
- How can I set the language independently for each user?
- How can I change the default language at the prompt when I need to 
enter the credentials (lightdm-greeter, I believe)?

All of this is on a Debian 9 installation with the packages 'debian 
desktop environment' and 'MATE' ('1' and '6').
I'd like to do this using the command line, that way, the users don't 
have to do it themself and I don't need to logg into each one.

--
John Doe