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.

Oleg Butuzov

For WP 2.3 – 2.5 you should user 1.3.* version
WP_Multilingual is extension that brings WordPress multilingual support. With it’s help you can publish more that in one language at a time. Its easy, cheap and simple way to use multilingual advantages of blogging.
With this plugin you can manage your Posts/Pages/Categories/Blogroll Links/Tags in multilingual mode.
Plugin also allow you to localize theme relate to Selected language, and localize Wp-admin in your language. 
Plugin Localized in
<ul>
<li>Russian</li>
<li>Ukrainian</li>
<li>English</li>
<li>Franch</li>
<li>Italian</li>
<li>Greek</li>
<li>Spanish</li>
<li>Czech</li>
<li>Catalan</li>
<li>Herbew</li>
<li>Deutsch</li>
<li>Dutch (Holland or Netherlands)</li>
<li>Arabian</li>
<li>Polish</li>
</ul>

WP_Multilingual

WP_Multilingual is extension that brings WordPress multilingual support. With it's help you can publish more that in one language at a time.

Disable Login Language Selector vs WP_Multilingual