In Special:Translate, the div that wraps most of the translate interface (.tux-messagelist) has some metadata about the current translation going on. When translating into nb, it looks like this:

<div class="row tux-messagelist" data-grouptype="RecentAdditionsMessageGroup" data-sourcelangcode="en" data-sourcelangdir="ltr" data-targetlangcode="nb" data-targetlangdir="ltr">

However, when you change the language using the language selector in the top right corner, the data-targetlangcode and data-targetlangdir attributes aren't updated. @Tacsipacsi
mentioned in this thread that directionality in the edit box also doesn't update if you switch between LTR and RTL languages, so there are probably more attributes affected by this. (Edit: This seems to have been fixed by @Nikerabbit back in September.)
List of steps to reproduce (step by step, including full links if applicable):

• Go to https://translatewiki.net/wiki/Special:Translate?group=%21additions&language=nb&filter=%21translated&action=translate
• Using the language selector in the top right, switch to another language.

What happens?:
The attributes on .tux-messagelist stay the same, not reflecting the fact that you are now no longer translating into nb.

What should have happened instead?:
The data-targetlangcode and data-targetlangdir attributes should be updated.

Software version (if not a Wikimedia wiki), browser information, screenshots, other information, etc:
MediaWiki 1.38.0-alpha, Translate 2021-10-25