dreamwinner

This plugin lets you toggle between your native and development language. 
It will help you to find the necessary line in the program code faster. 
Useful when you develop a plugin or perform language debugging. 
This plugin’s button may be found on admin’s dashboard and you can easy switch language anywhere, on any page.
<h3>Note</h3>
You can switch language only if: 
1) you have your native language files .PO and .MO installed in your WordPress CMS 
2) your native language available in your WordPress CMS and your browser can render it

Easy Language Switcher

Switch between English development language and native user language.

wp-ave.com

Hides language switcher feature from login screens introduced in WordPress 5.9
Features:
<ul>
<li>Simply activate this plugin to hide language switcher feature from login screens introduced in WordPress 5.9</li>
</ul>

Hide Language Switcher

Hides language switcher feature from login screens introduced in WordPress 5.9

Easy Language Switcher vs Hide Language Switcher