set_custom_path example

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

    $wp_textdomain_registry->set_custom_path( $domain, $path );

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

/** * Loads the translated strings for a plugin residing in the mu-plugins directory. * * @since 3.0.0 * @since 4.6.0 The function now tries to load the .mo file from the languages directory first. * * @global WP_Textdomain_Registry $wp_textdomain_registry WordPress Textdomain Registry. * * @param string $domain Text domain. Unique identifier for retrieving translated strings. * @param string $mu_plugin_rel_path Optional. Relative to `WPMU_PLUGIN_DIR` directory in which the .mo * file resides. Default empty string. * @return bool True when textdomain is successfully loaded, false otherwise. */