In new Vector skin, an action button is placed at the article header which should open a language list of articles in other project languages. It seems that there are two different modes for (1) users not logged-in and (2) users logged-in. In the first case, a more comprehensive dialogue is shown with an input line for language search and preferences options. This seems working well.
In the second case a reduced dialogue with a language list and an "edit links" link occur. But this is often not working. If you press on the language button then in many cases nothing happens. You can predict if the list is shown or not. If you are pressing the button and the background color remains white and the border is dotted then a list will be opened. If you are pressing the button and the background color becomes gray and the border is solid then the list will not be opened. It seems that there is no event listener or a listener not working well.
No error report in this respect is given at the console. It seems that in case of the erroneous behavior an error message like "Uncaught ReferenceError: jQuery36006843335910066104_1626853792556 is not defined" (api.php:1:5) occurs.
I observed this erroneous behavior both on Wikipedias and Wikivoyages.