Sets the WordPress locale to MLP's current blog language, allowing custom/different language files to be loaded

mlp-set-locale.php

/**
 * Plugin Name: MLP Load Language Files
 * Description: Sets the WP locale to MLP's current blog language so that different language files can be loaded
 * Author:      Inpsyde GmbH
 * Author URI:  http://inpsyde.com
 * Text Domain: multilingualpress
 * Domain Path: /languages
 * License:     GPLv3
 */


if ( ! is_admin() ) {
	add_action( 'inpsyde_mlp_loaded', 'inpsyde_load_mlp_locale_init' );
}

/**
 * Load up the plugin
 */
function inpsyde_load_mlp_locale_init() {

	$load_locale = new Mlp_Addon_Load_Locale();
	$load_locale->init();
}

class  Mlp_Addon_Load_Locale {

	/**
	 * Initialize this class
	 */
	public function init() {

		add_filter( 'locale', array( $this, 'load_mlp_locale' ) );
	}

	/**
	 * Replace the WP locale based on MLP's current language
	 * 
	 * @param $locale
	 *
	 * @return mixed
	 * @wp-hook locale
	 */
	public function load_mlp_locale( $locale ) {

		$api = apply_filters( 'mlp_language_api', NULL );
		if ( ! ( $api instanceof Mlp_Language_Api_Interface ) ) { // for IDE support
			return $locale;
		}

		$languages        = get_site_option( 'inpsyde_multilingual' );
		$current_language = $languages[ get_current_blog_id() ];

		if ( ! empty( $current_language[ 'lang' ] ) ) {
			return $current_language[ 'lang' ];

		}

		return $locale;
	}
}