By enabling the user to select their preferred locale when editing the profile, we allow for greater personalization of the WordPress admin and therefore a better user experience.
The back end will be displayed in the user's individual locale while the locale used on the front end equals the one set for the whole site. If the user didn't specify a locale, the site's locale will be used as a fallback. The new `locale` property of the `WP_User` class can be used to retrieve the user's locale setting.
Props ocean90, ipm-frommen, swissspidy.
Fixes#29783.
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git-svn-id: http://core.svn.wordpress.org/trunk@38648 1a063a9b-81f0-0310-95a4-ce76da25c4cd
