With the introduction of user-specific languages in [38705] it's necessary to be able to switch translations on the fly. For example emails should be sent in the language of the recipient and not the one of the current user.
This introduces a new `WP_Locale_Switcher` class which is used for switching locales and translations. It holds the stack of locales whenever `switch_to_locale( $locale )` is called. With `restore_previous_locale()` you can restore the previous locale. `restore_current_locale()` empties the stack and sets the locale back to the initial value.
`switch_to_locale()` is added to most of core's email functions, either with the value of `get_locale()` (site language) or `get_user_locale()` (user language with fallback to site language).
Props yoavf, tfrommen, swissspidy, pbearne, ocean90.
See #29783.
Fixes#26511.
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git-svn-id: http://core.svn.wordpress.org/trunk@38904 1a063a9b-81f0-0310-95a4-ce76da25c4cd
