Force back-end language for all users

In the functions.php file add this:

<?php
add_filter('locale', function($locale) {
    return is_admin() ? 'en_US' : $locale;
});

<?php
add_filter('locale', function($locale) {
    return is_admin() ? 'en_US' : $locale;
});

Useful when you develop in English but the site is in another language.