This bug can be fixed by overriding $this->icons['left'] and $this->icons['right'] and adding calls to format_string() in question/category_class.php as described in category_class-patch.txt.
Alternatively format_string() calls could be added to lib/listlib.php like in listlib-patch.txt, but I'm not sure about any site-wide negative effects the patch could have.

0. Ensure the multilang filter is turned on, and applies to content and headings.

1. Create a new quiz containing multilang strings in quiz name,
for example: <span lang="en" class="multilang">English name</span><span lang="fr" class="multilang">Nom Francaise</span>

2. In the quiz settings block, select Question bank -> Categories. Ensure the quiz name, category name and description are all displayed properly (i.e. only the current lang shown).

3. Check the other three question bank sub-links, and again ensure that everywhere the category name is displayed, it is displayed correctly.