OzTheGreat

Adds a multiple select box to every non-admin user profile that allows admins to 
select which languages that user can see / edit in the wp-admin.
Languages that a user cannot see are hidden. If they try to switch surreptitiously 
it will throw an error message.
This plugin REQUIRES WPML and is useless without it.

WPML Editor Languages

Allows admins to restrict users to specific languages in the WordPress admin

Rick Curran

WordPress 5.9 added a new language selector to allow users to switch languages from a dropdown on the login screen if more than one language is enabled on your WordPress installation. This plugin provides a quick and easy way to prevent this language selector from appearing.
It is a very simple plugin which is provided for ease of use: you need only to install and activate the plugin and it will prevent the language dropdown from appearing on the login screen of your site.
Alternatively, you can also achieve the same effect by adding the following line of code to the <code>functions.php</code> file in your theme:
<pre><code>add_filter( 'login_display_language_dropdown', '__return_false' );
</code></pre>

Disable Login Language Selector

Remove the language selector from the login screen if you have more than one language enabled in WordPress 5.9 and later.

WPML Editor Languages vs Disable Login Language Selector