The button to show all languages in a compact language list is no longer working on English Wikipedia. For example, clicking on "12 more" in the language list at https://en.wikipedia.org/wiki/Wikipedia:Teahouse does nothing. See also https://en.wikipedia.org/w/index.php?title=Wikipedia:Teahouse&oldid=858357806 (section "language links" at bottom) for similar reports from other users.
The "Use a compact language list" setting in Special:Preferences --> "Appearances" is checked.
System settings: Vector skin, Firefox 52.9 (possibly also on Chrome, as reported by another user - see Teahouse thread link).