Introduces new `determine_locale()` function for deciding the proper locale to use for a response. Default value is `get_user_locale()` in the admin, and `get_locale()` on the frontend. Because REST API requests are considered frontend requests, `?_locale=user` can be used to render the response in the user's locale.
Also updates `wp-login.php?wp_lang` implementation to benefit from this abstraction.
Merges [43776] from the 5.0 branch to trunk.
Props flixos90, mnelson4, swissspidy, TimothyBlynJacobs.
Fixes#44758.
git-svn-id: https://develop.svn.wordpress.org/trunk@44134 602fd350-edb4-49c9-b593-d223f7449a82