This new function makes it easier to switch to a specific user’s locale by reducing duplicate code and storing the user’s ID as additional context for plugins to consume. Existing usage of `switch_to_locale()` in core has been replaced with `switch_to_user_locale()` where appropriate.
Also, this change ensures `WP_Locale_Switcher` properly filters `determine_locale` so that anyyone using the `determine_locale()` function will get the correct locale information when switching is in effect.
Props costdev.
Fixes#57123.
See #26511.
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git-svn-id: http://core.svn.wordpress.org/trunk@54694 1a063a9b-81f0-0310-95a4-ce76da25c4cd
