Dominik Schilling

This plugin helps you to declutter your WordPress login screen by removing the language switcher which has been <a href="https://make.wordpress.org/core/2021/12/20/introducing-new-language-switcher-on-the-login-screen-in-wp-5-9/" rel="nofollow ugc">introduced in WordPress 5.9</a>.
Removing the dropdown makes sense when you only have one additional language installed and don’t intend to change back to English for example. In this case you can simply install and activate the plugin. There are no futher settings.
<h3>What does this plugin do?</h3>
The plugin uses the default <a href="https://developer.wordpress.org/plugins/hooks/" rel="nofollow ugc">WordPress Hooks API</a> and disables the language selector with the following one-liner:
<pre><code>add_filter( 'login_display_language_dropdown', '__return_false' );
</code></pre>

Disable Login Language Switcher

Does what it says on the tin.

Removes the language dropdown from the login screen.

unclego

Lets you have your backend administration panel in english or any installed language, even if the rest of your blog is translated into another language. Language preferences can be set per user basis in user profile screen.
Tested on MultiSite.
Deals with frontend ajax requests
Notice: 
After release of WordPress 4.7 this plugin is a bit obsolete.
In WordPress 4.7 users are able to select their preferred locale (language) when editing their profile. Plugin add exact same functionality to pre 4.7 WordPress.
I still maintain it for people with outdated WordPress sites. And the plugin is still usefull for sites with outdated plugins that not obey get_user_locale(). 
<a href="https://make.wordpress.org/core/2016/11/07/user-admin-languages-and-locale-switching-in-4-7/" rel="nofollow ugc">More info</a>

Admin Language Per User

a must have for multilingual sites

Switch your backend administration panel in any installed language per user basis.

Disable Login Language Switcher vs Admin Language Per User