Add a language chooser to the metabox

[terw-dan]

Since we are supporting more and more languages for the readability scores, I think it would be a good idea to add a language chooser to the metabox.

Currently we assume the language of the admin is the same language the post is written in, but this doesn't have to be true. It probably will be for most users, but an option to override this would be useful.

We only need it for the readability tab, since this is language dependent. It could look something like this

In stead of using the current locale from the admin and pushing that to the paper, we can use the value from the dropdown to set in the paper. So updating the value from the select box should put that in the paper, and we could save this per post in the database, so you don't have to set it every time you create a new post.

We could even make a setting in the Yoast SEO settings to put a default language in for posts if this is desired.

[afercia]

Related: this new WP feature that will probably land in 4.7: https://core.trac.wordpress.org/ticket/29783

[moorscode]

Having a user language helps to facilitate a more specific need to have language experts on the site. Although a single user can also write in multiple languages.

When user [A] with language [1] selected visits a post of user [B] with language [2] you still want to have the checks set for the language the document is initially written in.

This is also a good indicator for the language tag on the front-end itself.

The placement of the language selection might be a discussion point on itself, because it is a "document" setting and not a "readability" or keyword specific thing. Thus it might even belong in WordPress core in page meta.

[atimmer]

Related #6053