When getting e-mails after subscribed to a forum, there are two problems when multilang is used:

Problem 1:
The e-mail subject / title contains the course shortname. If this shortname has multilang-tags, these are all displayed and the mail subject looks like: "<span lang="xx" class="multilang">text</span><span lang="xx" class="multilang">text...</span>"

Responsible is line 612 of /mod/forum/lib.php:
$postsubject = "$course->shortname: ".format_string($post->subject,true);

Problem 2:
The content of the mail also contains the course shortname. This is also not displayed correctly (all languages one after another).

To fix the second problem, go to /mod/forum/lib.php line 1080 and add a format_string():
'<a target="_blank" href="'.$CFG->wwwroot.'/course/view.php?id='.$course->id.'">'.format_string($course->shortname).'</a> » '.