The links to individual languages in the <languages /> tag from the page translation feature should have lang and or hreflang attributes. This would make it easier to e.g. apply CSS to certain languages, for example to highlight the official languages used on a wiki, or in user stylesheets to apply custom styling to languages of the user's choosing. The same should also apply to the span tag for the current language.
One practical usecase where I could've used this: this diff on no.wikimedia.org, which is a hack until T156589 is fixed.