Steps to reproduce:

• Set a course language to English.
• Set a user language to Portuguese (or any other language).
• Assign this user as a teacher for this course.
• In the course select the user list .
• Choose any action.could it be "Send a message to selected users".
• The message screen will have the user language and not the course language.

We add these two lines in the user/action_redir.php file and now the course language is presented .

//CUSTOM CODE.
$courselang = $DB->get_field('course','lang',array('id'=>$id));
$SESSION->lang = $courselang;