Just ran an accessibility audit using Lighthouse.
Among other things, Lighthouse suggests to add a lang attribute to the document, here's why: (from the Lighthouse report)
If a page doesn't specify a lang attribute, a screen reader assumes that the page is in the default language that the user chose when setting up the screen reader. If the page isn't actually in the default language, then the screen reader might not announce the page's text correctly.
Seems like a quick win. Any reason not to?