Language switching methods akin to WordPress's blog switching methods

[goncaloasimoes]

In order to make code like this:

add_filter( 'ubb_current_lang', $fn = fn () => $lang );
// Perform something in another language.
remove_filter( 'ubb_current_lang', $fn );

look better, we should add a pair of methods:

• switch_to_language( string $language )
  -restore_language()

akin to the ones used by WordPress to switch blogs.