The bidi.css file contains a 250px left margin which is added to the dt list inner class on small screens, (less than 700px - responsive mode) this stops the subforums from displaying correctly when using an RTL language.

The LTR version doesn't have this rule so it must have been left over from changes in the past.

I have added both LTR and RTL screen shots to show the differences.

File:

bidi.css

Delete:

    .rtl ul.topiclist.forums dt .list-inner {

        margin-left: 250px;

    }