Nastavení jiného jazyka pro web (frontend) a jiného pro administraci

User Language Switch
https://wordpress.org/plugins/user-language-switch/

Allows backend users to set the language displayed in the back-end and front-end of your site. It also allows to translate pages and posts. Define one language for admin and the other for theme. Don’t define WPLANG in your wp-config.php file.

A nebo:

// setup one language for admin and the other for theme
// must be called before load_theme_textdomain()
function set_my_locale($locale) {
	
	$locale = ( is_admin() ) ? "cz_CS" : "en_US";
	setlocale(LC_ALL, $local );
	return $locale;

}
add_filter( 'locale', 'set_my_locale' );