Source

File: wp-includes/l10n.php

function load_textdomain( $domain, $mofile ) {
	global $l10n, $l10n_unloaded;

	$l10n_unloaded = (array) $l10n_unloaded;

	/**
	 * Filters whether to override the .mo file loading.
	 *
	 * @since 2.9.0
	 *
	 * @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.
	 */
	$plugin_override = apply_filters( 'override_load_textdomain', false, $domain, $mofile );

	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 ( ! is_readable( $mofile ) ) {
		return false;
	}

	$mo = new MO();
	if ( ! $mo->import_from_file( $mofile ) ) {
		return false;
	}

	if ( isset( $l10n[ $domain ] ) ) {
		$mo->merge_with( $l10n[ $domain ] );
	}

	unset( $l10n_unloaded[ $domain ] );

	$l10n[ $domain ] = &$mo;

	return true;
}