load_textdomain() │ WP 1.5.0

load_textdomain( $domain, $mofile, $locale );

// here the .mo file must be in the `lang` folder, which is in 
// the folder that contains the file in which this (string - code) is called.
// Connect the existing .mo file (file name: ru_RU.mo or other, depending on the locale)
add_action( 'plugins_loaded', 'load_my_textdomain' );

function load_my_textdomain(){
	$mo_file_path = dirname(__FILE__) . '/lang/'. determine_locale() . '.mo';

	load_textdomain( 'mydomain', $mo_file_path );
}

<?php _e( 'book', 'mydomain' ); ?>

if ( is_textdomain_loaded( $plugin ) ) {
	unload_textdomain( $plugin );
}

$mofile = sprintf( '%s-%s.mo', $plugin, $locale );

// check the installation language path first.
$domain_path = path_join( WP_LANG_DIR, 'plugins' );
$loaded = load_textdomain( $plugin, path_join( $domain_path, $mofile ) );

// else, check the plugin language folder.
if ( ! $loaded ) { 
	$domain_path = path_join( WP_PLUGIN_DIR, "{$plugin}/languages" );
	load_textdomain( $plugin, path_join( $domain_path, $mofile ) );
}

Notes

• Global. MO[]. $l10n An array of all currently loaded text domains.
• Global. MO[]. $l10n_unloaded An array of all text domains that have been unloaded again.
• Global. WP_Textdomain_Registry. $wp_textdomain_registry WordPress Textdomain Registry.

Changelog

load_textdomain() load textdomain code ^{WP 6.5.2}

function load_textdomain( $domain, $mofile, $locale = null ) {
	/** @var WP_Textdomain_Registry $wp_textdomain_registry */
	global $l10n, $l10n_unloaded, $wp_textdomain_registry;

	$l10n_unloaded = (array) $l10n_unloaded;

	/**
	 * Filters whether to short-circuit loading .mo file.
	 *
	 * Returning a non-null value from the filter will effectively short-circuit
	 * the loading, returning the passed value instead.
	 *
	 * @since 6.3.0
	 *
	 * @param bool|null   $loaded The result of loading a .mo file. Default null.
	 * @param string      $domain Text domain. Unique identifier for retrieving translated strings.
	 * @param string      $mofile Path to the MO file.
	 * @param string|null $locale Locale.
	 */
	$loaded = apply_filters( 'pre_load_textdomain', null, $domain, $mofile, $locale );
	if ( null !== $loaded ) {
		if ( true === $loaded ) {
			unset( $l10n_unloaded[ $domain ] );
		}

		return $loaded;
	}

	/**
	 * Filters whether to override the .mo file loading.
	 *
	 * @since 2.9.0
	 * @since 6.2.0 Added the `$locale` parameter.
	 *
	 * @param bool        $override Whether to override the .mo file loading. Default false.
	 * @param string      $domain   Text domain. Unique identifier for retrieving translated strings.
	 * @param string      $mofile   Path to the MO file.
	 * @param string|null $locale   Locale.
	 */
	$plugin_override = apply_filters( 'override_load_textdomain', false, $domain, $mofile, $locale );

	if ( true === (bool) $plugin_override ) {
		unset( $l10n_unloaded[ $domain ] );

		return true;
	}

	/**
	 * Fires before the MO translation file is loaded.
	 *
	 * @since 2.9.0
	 *
	 * @param string $domain Text domain. Unique identifier for retrieving translated strings.
	 * @param string $mofile Path to the .mo file.
	 */
	do_action( 'load_textdomain', $domain, $mofile );

	/**
	 * Filters MO file path for loading translations for a specific text domain.
	 *
	 * @since 2.9.0
	 *
	 * @param string $mofile Path to the MO file.
	 * @param string $domain Text domain. Unique identifier for retrieving translated strings.
	 */
	$mofile = apply_filters( 'load_textdomain_mofile', $mofile, $domain );

	if ( ! $locale ) {
		$locale = determine_locale();
	}

	$i18n_controller = WP_Translation_Controller::get_instance();

	// Ensures the correct locale is set as the current one, in case it was filtered.
	$i18n_controller->set_locale( $locale );

	/**
	 * Filters the preferred file format for translation files.
	 *
	 * Can be used to disable the use of PHP files for translations.
	 *
	 * @since 6.5.0
	 *
	 * @param string $preferred_format Preferred file format. Possible values: 'php', 'mo'. Default: 'php'.
	 * @param string $domain           The text domain.
	 */
	$preferred_format = apply_filters( 'translation_file_format', 'php', $domain );
	if ( ! in_array( $preferred_format, array( 'php', 'mo' ), true ) ) {
		$preferred_format = 'php';
	}

	$translation_files = array();

	if ( 'mo' !== $preferred_format ) {
		$translation_files[] = substr_replace( $mofile, ".l10n.$preferred_format", - strlen( '.mo' ) );
	}

	$translation_files[] = $mofile;

	foreach ( $translation_files as $file ) {
		/**
		 * Filters the file path for loading translations for the given text domain.
		 *
		 * Similar to the {@see 'load_textdomain_mofile'} filter with the difference that
		 * the file path could be for an MO or PHP file.
		 *
		 * @since 6.5.0
		 *
		 * @param string $file   Path to the translation file to load.
		 * @param string $domain The text domain.
		 */
		$file = (string) apply_filters( 'load_translation_file', $file, $domain );

		$success = $i18n_controller->load_file( $file, $domain, $locale );

		if ( $success ) {
			if ( isset( $l10n[ $domain ] ) && $l10n[ $domain ] instanceof MO ) {
				$i18n_controller->load_file( $l10n[ $domain ]->get_filename(), $domain, $locale );
			}

			// Unset NOOP_Translations reference in get_translations_for_domain().
			unset( $l10n[ $domain ] );

			$l10n[ $domain ] = new WP_Translations( $i18n_controller, $domain );

			$wp_textdomain_registry->set( $domain, $locale, dirname( $file ) );

			return true;
		}
	}

	return false;
}

Related Functions

textdomain (mo po files)

localize (l10n i18n Internationalization localization)

Localization