WordPress 5.9 introduced a language switcher dropdown under the login form on wp-login.php. I didn’t see that feature in the news, so I missed it for a while. I don’t need it and most of my sites that already have multilingual content don’t need it, so I had to check how I could remove it. The direct solution with a single line of code in the theme’s functions.php is this one:
//Remove language switcher from wp-login.php, introduced in WP 5.9
add_filter( 'login_display_language_dropdown', '__return_false');