https://www.w3.org/WAI/standards-guidelines/act/rules/b5c3f8/ and
https://dequeuniversity.com/rules/axe-devtools/3.1/html-has-lang
require a lang attribute. It is only possible to set one value.
If the user is logged in, set their "Translation" which they set on `/settings/panel/language/`, else set to English.
Disadvantage: The translation might be so incomplete that they'll stick get most strings in English while their screenreader software expects a different language.
See T16072