Inpsyde\MultilingualPress\Common\Admin\SitesListTableColumn::add PHP Method

    public function add(array $columns)
    {
        if (is_callable($this->add_callback)) {
            return (array) call_user_func([$this, 'add_callback'], $columns, $this->id, $this->name);
        }
        return array_merge($columns, [$this->id => $this->name]);
    }