Language for course completed message

As a student, log in to a course in which:

• An activity has been configured for self-completion
• Course completion tracking has been configured and includes this activity
• Select the activity as complete.
  On course completion, a "course completed" message is sent to the student.

Expected behavior
Moodle sends the course completed message in the user's preferred language.

Actual behavior
Moodle sends the course completed message in the same language for all users regardless of their preferred language.

Details
The course completed message contains the strings "coursecompleted" and "coursecompletedmessage".
Moodle sends the message in the language defined by the site's "Default language" (lang) setting.
The message should better be sent in the user's preferred language.

Proposed fix
In the function mark_complete() in completion/completion_completion.php, get strings for $messagesubject and $messagebody according to the user's preferred language.