This is not an RTL bug :)
Try going to https://simple.wikipedia.beta.wmflabs.org/wiki/Florida?useskin=timeless . In the Timeless skin the interlanguage links appear in a sidebar on the right-hand side of the screen. Clicking the "136 more" (languages) button opens the language selection panel on the right-hand side of the button, so most of it is not actually visible. This happens because the common Vector and Monobook skins show the languages list on the left-hand sidebar, so it was assumed that the panel should always be on the right.
In the Timeless skin, and possibly in others, the panel should open itself to the left. To make ULS in general more robust, there should be some algorithm for automatic identification of the best side on which the panel should be shown.
At the moment this is not a high-priority issue for ULS and not a blocker for wider Compact Language Links deployment on WIkimedia sites because this doesn't affect any skin used on Wikimedia sites. However, this is a blocker for deploying Timeless on Wikimedia sites (T154371) if this ever happens, and this should be fixed for better general robustness for ULS.
(This description assumes an LTR wiki. It affects RTL wikis the same way, but the words "left" and "right" are flipped everywhere. When fixing, it must be tested in both RTL and LTR wikis.)