This have an impact in term of accessibility, as user-side software will rely on this metadata to render the page content.

Also, server side, some special features like {{formatnum:123345.678}} depends on the locale of the content.

One may wrap the whole wikitext in a div with the good lang attribute, but this is not very user-friendly. Also for the case of beta.wikiversity, most pages are –hopefully– intended to end up on a specific language domain. Sure you can plane that in migration process, but not having to would be preferable.

One possibility would be to use categories to indicate the main language of a page.