The interwiki section of the sidebar is missing in the modern skin
Initial reports suggest this just started happening, able to easily reproduce, example link of a random page:
https://en.wikipedia.org/wiki/Wikipedia?useskin=modern
Originally reported on enwiki: https://en.wikipedia.org/w/index.php?title=Wikipedia:Village_pump_(technical)&oldid=984558520#What_happens_to_Modern_skin?