If you are seeing a language switcher on the log-in page of your site, please be advised that this is a new WP 5.9 feature. At this time, there isn't an option for the admin/owner of the website to hide that. Please note that CSS code will not be able to hide it. 

You will be able to hide the language switcher by following the guidelines in this article:

https://nimb.ws/x72dLq

from here:

https://make.wordpress.org/core/2021/12/20/introducing-new-language-switcher-on-the-login-screen-in-wp-5-9/