I tried validating the HTML of my website, and it failed on the Site-level language switch. The ul tag used to list the available languages is a block-level element, and this is nested within various span elements. As span is a phrasing element, it also should only contain phrasing elements. Either the ul should be replaced with something else, or the parent tags should be changed to something like a div.

If you would prefer me to make a PR for this I can do so.