I noticed this in Nightly today (proton prefs enabled if that matters) - when I went to add a 6th langpack, it didn't work.

Retracing my steps, it's because once you add the 6th item to the list, it expands and pushes the "submit" button out of view. I missed that it wasn't there and just closed the modal thinking I was done.

Imagine this is a relatively edge case, but figured I'd file nonetheless.

STR:

1. About:preferences
2. Go to Language and Appearance.
3. Add enough langpacks to trigger overflow (for me, it was 5)
4. Now, add one more langpack. Watch what happens when you click the "add" button. It knocks the submit button out of view.

Here's a video of this in action.