Warmup post languages in WP_Query

[goncaloasimoes]

When a WP_Query fetches X amount of posts. If there is a processing in the frontend of the queried posts to fetch their permalinks, each of them will have their language fetched. LangInterface::get_post_language has a transient, but if there is no transient for the posts, there will be a lot of different separate queries to get each post's language.

In order to combat this, upon a successful WP_Query for posts, we can warmup these transients with the post languages by doing one query for all of them.