How to create a Simple WordPress Language Switcher | https://www.ibenic.com/how-to-create-a-simple-wordpress-language-switcher/

plugin-2.php

<?php

// ... previous code

class SimpleWordPressLanguageSwitcher {
   // ... previous code


   /**
    * Return the correct locale based on the $_GET parameter.
    */
   public function get_locale_to_set() {
		if ( isset( $_GET['locale'] ) ) {
			switch ( $_GET['locale'] ) {
				case 'en':
					return 'en_US';
					break;
				case 'hr':
					return 'hr';
					break;

			}
		}

		return false;
	}
}

plugin-3.php

<?php

// ... previous code

class SimpleWordPressLanguageSwitcher {
   // ... previous code

   
   public function set_locale() {
     if ( $this->get_locale_to_set() ) {
		$locale = $this->get_locale_to_set();
		setcookie( 'my_language', $locale, time() + YEAR_IN_SECONDS, COOKIEPATH, COOKIE_DOMAIN );
		switch_to_locale( $locale ); // Switching directly just in case.
		return;
	 }
   }
}

plugin-4.php

<?php

// ... previous code

class SimpleWordPressLanguageSwitcher {
   // ... previous code

   
   public function get_locale( $locale ) {
		if ( $this->get_locale_to_set() ) {
			$locale = $this->get_locale_to_set();
		} else {
			$locale = isset( $_COOKIE['my_language'] ) ? $_COOKIE['my_language'] : $locale;
		}

		return $locale;
	}
}

plugin.php

<?php

/**
 * Plugin Name: Simple WordPress Language Switcher
 * Version: 1.0.0.
 */

if ( ! defined( 'ABSPATH' ) ) { return; }

class SimpleWordPressLanguageSwitcher {
   
  public function __construct() {
		add_action( 'init', [ $this, 'set_locale' ] );
		add_action( 'pre_determine_locale',[ $this, 'get_locale' ] );
  }
  
  /**
   * Set the Locale in Cookie if it exists.
   */
  public function set_locale() {
     
  }


  /**
   * Get the Locale from cookie if it exists.
   */
  public function get_locale( $locale ) {
    

    return $locale;
  }

}

new SimpleWordPressLanguageSwitcher();