load_plugin_textdomain( string $domain, string $deprecated = false, string $plugin_rel_path = false )

function load_plugin_textdomain( $domain, $deprecated = false, $plugin_rel_path = false ) {
	/**
	 * Filters a plugin's locale.
	 *
	 * @since WP-3.0.0
	 *
	 * @param string $locale The plugin's current locale.
	 * @param string $domain Text domain. Unique identifier for retrieving translated strings.
	 */
	$locale = apply_filters( 'plugin_locale', is_admin() ? get_user_locale() : get_locale(), $domain );

	$mofile = $domain . '-' . $locale . '.mo';

	// Try to load from the languages directory first.
	if ( load_textdomain( $domain, WP_LANG_DIR . '/plugins/' . $mofile ) ) {
		return true;
	}

	if ( false !== $plugin_rel_path ) {
		$path = WP_PLUGIN_DIR . '/' . trim( $plugin_rel_path, '/' );
	} elseif ( false !== $deprecated ) {
		_deprecated_argument( __FUNCTION__, 'WP-2.7.0' );
		$path = ABSPATH . trim( $deprecated, '/' );
	} else {
		$path = WP_PLUGIN_DIR;
	}

	return load_textdomain( $domain, $path . '/' . $mofile );
}