Akshar Soft Solutions

This plugin lets users change their administration language to native English (en_US locale).
This is useful during site development and for people more accustomed to the english administration panel, even if your site 
is in another language. (The frontend will still use the native language.)
This plugin is fully compatible with WooCommerce and can correctly identify and translate frontend / backend AJAX requests. 
If you are using this plugin with Advanced Custom Fields, please move the as-english-wp-admin.php file to the /wp-content/mu-plugins/ folder 
(create it if it does not exist). This is an ACF limitation.
This plugin is developer friendly and small (~200 lines of code). Check the FAQ for customization examples.
Usage
Basic usage
Once you have installed and activated the plugin, navigate to any admin page and check the top admin bar. A button will 
display your current locale. If you click on it, the admin will change to English locale (en_US). To switch back, 
press the button again.
<h3>Requirements</h3>
<ul>
<li>PHP 5.3 or higher</li>
</ul>
<h3>Translations</h3>
<ul>
<li>None</li>
</ul>

AS English Admin

Lets users change their administration language to English

martinjuhasz

Lets you choose your Backend Language for your complete Backend, also if your Frontend is using another Language (including Multisite). 
You can choose from any language you installed. For a list of availible languages, check <a href="https://codex.wordpress.org/WordPress_in_Your_Language" rel="nofollow ugc">“WordPress_in_Your_Language”</a> or create your own files.

One Backend Language

Lets you choose your Backend Language for your complete Backend, also if your Frontend is using another Lang (including Multisite)

AS English Admin vs One Backend Language