Autoincrement primary key in CREATE TABLE ... AS SELECT
CREATE TABLE `user_mv` (id INT AUTO_INCREMENT PRIMARY KEY) SELECT `user`.`firstname` as 
   `firstname`,
   `user`.`lastname` as `lastname`,
   `user`.`lang` as `lang`,
   `user`.`name` as `user_name`,
   `group`.`name` as `group_name`
from `user`
  inner join `user_groups` on (`user`.`user_id`=`user_groups`.`user_id`)
  left  join `group` on (`group`.`group_id`=`user_groups`.`group_id`)
where `user`.`lang`=`group`.`lang`;


As long as you don't have an 'id' column in your SELECT, this appears to do exactly what you want.  The columns in the select will be appended to the right of the columns you declare.

http://dev.mysql.com/doc/refman/5.6/en/create-table-select.html (also in previous versions).