Make dropdown language block cacheable

Postponed on 📌 Make language switcher block cacheable Postponed

I had this annoying thing in my project when sometimes when you open a page the dropbutton would display as open so you see all the language links but the triangle down icon was missing, and sometimes it would work, showing only the current language with the dropdown button (triangle down icon) next to it. Refreshing the page is like playing russian roulette on whether the dropdown language would work or not. about 3/5 of the time it would not work correctly.

When it does not work correctly, the wrapper looks like this:
<div class="dropbutton-wrapper">
Also the <li> elements are missing classes like dropbutton-action

When it works correctly, the wrapper looks like this:
<div class="dropbutton-wrapper dropbutton-multiple" data-drupal-ajax-container="" data-once="dropbutton">
and the li elements inside have their correct classes.

Adding this (applied the MR on 4.0.3) fixed this issue for me.