The course shortnames aren't affected by multilang filter in grade overview mode. Not only admins but also students will see this when taking a look into the gradebook.

Reproduction:

(1) Create a course using multilang filter in its name/shortname and assign a user to this course
(2) On the platform, go to the course and choose "Grades"
(3) In the dropdown boxes, choose "Overview report" on the left and select the user on the right side
(4) You can now see the course shortname showing all languages one after another

Fix instructions:

(1) Open /grades/report/overview/lib.php
(2) Go to line 122 and add a "format_string()" to "$course->shortname" .....the line should now look like this: $courselink = '<a href="'.$CFG->wwwroot.'/grade/report/user/index.php?id='.$course->id.'&userid='.$this->user->id.'">'.format_string($course->shortname).'</a>';
(3) Refresh the browser, now it should work!

Attached: screenshot before and after the fix

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Concerning multilang and course shortnames another improvement at this point: The course shortname is limited to 100 chars. Using multilang filter, this could lead to a problem, esp. when using more than two languages. This has to be changed in /course/edit_form.php and additionally in the database (table mdl_course -> shortname).