diff -r 490d5cc509ed -r cf61fcea0001 wp/wp-includes/class-wp-locale-switcher.php
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/wp/wp-includes/class-wp-locale-switcher.php	Mon Oct 14 17:39:30 2019 +0200
@@ -0,0 +1,235 @@
+<?php
+/**
+ * Locale API: WP_Locale_Switcher class
+ *
+ * @package WordPress
+ * @subpackage i18n
+ * @since 4.7.0
+ */
+
+/**
+ * Core class used for switching locales.
+ *
+ * @since 4.7.0
+ */
+class WP_Locale_Switcher {
+	/**
+	 * Locale stack.
+	 *
+	 * @since 4.7.0
+	 * @var string[]
+	 */
+	private $locales = array();
+
+	/**
+	 * Original locale.
+	 *
+	 * @since 4.7.0
+	 * @var string
+	 */
+	private $original_locale;
+
+	/**
+	 * Holds all available languages.
+	 *
+	 * @since 4.7.0
+	 * @var array An array of language codes (file names without the .mo extension).
+	 */
+	private $available_languages = array();
+
+	/**
+	 * Constructor.
+	 *
+	 * Stores the original locale as well as a list of all available languages.
+	 *
+	 * @since 4.7.0
+	 */
+	public function __construct() {
+		$this->original_locale     = is_admin() ? get_user_locale() : get_locale();
+		$this->available_languages = array_merge( array( 'en_US' ), get_available_languages() );
+	}
+
+	/**
+	 * Initializes the locale switcher.
+	 *
+	 * Hooks into the {@see 'locale'} filter to change the locale on the fly.
+	 */
+	public function init() {
+		add_filter( 'locale', array( $this, 'filter_locale' ) );
+	}
+
+	/**
+	 * Switches the translations according to the given locale.
+	 *
+	 * @since 4.7.0
+	 *
+	 * @param string $locale The locale to switch to.
+	 * @return bool True on success, false on failure.
+	 */
+	public function switch_to_locale( $locale ) {
+		$current_locale = is_admin() ? get_user_locale() : get_locale();
+		if ( $current_locale === $locale ) {
+			return false;
+		}
+
+		if ( ! in_array( $locale, $this->available_languages, true ) ) {
+			return false;
+		}
+
+		$this->locales[] = $locale;
+
+		$this->change_locale( $locale );
+
+		/**
+		 * Fires when the locale is switched.
+		 *
+		 * @since 4.7.0
+		 *
+		 * @param string $locale The new locale.
+		 */
+		do_action( 'switch_locale', $locale );
+
+		return true;
+	}
+
+	/**
+	 * Restores the translations according to the previous locale.
+	 *
+	 * @since 4.7.0
+	 *
+	 * @return string|false Locale on success, false on failure.
+	 */
+	public function restore_previous_locale() {
+		$previous_locale = array_pop( $this->locales );
+
+		if ( null === $previous_locale ) {
+			// The stack is empty, bail.
+			return false;
+		}
+
+		$locale = end( $this->locales );
+
+		if ( ! $locale ) {
+			// There's nothing left in the stack: go back to the original locale.
+			$locale = $this->original_locale;
+		}
+
+		$this->change_locale( $locale );
+
+		/**
+		 * Fires when the locale is restored to the previous one.
+		 *
+		 * @since 4.7.0
+		 *
+		 * @param string $locale          The new locale.
+		 * @param string $previous_locale The previous locale.
+		 */
+		do_action( 'restore_previous_locale', $locale, $previous_locale );
+
+		return $locale;
+	}
+
+	/**
+	 * Restores the translations according to the original locale.
+	 *
+	 * @since 4.7.0
+	 *
+	 * @return string|false Locale on success, false on failure.
+	 */
+	public function restore_current_locale() {
+		if ( empty( $this->locales ) ) {
+			return false;
+		}
+
+		$this->locales = array( $this->original_locale );
+
+		return $this->restore_previous_locale();
+	}
+
+	/**
+	 * Whether switch_to_locale() is in effect.
+	 *
+	 * @since 4.7.0
+	 *
+	 * @return bool True if the locale has been switched, false otherwise.
+	 */
+	public function is_switched() {
+		return ! empty( $this->locales );
+	}
+
+	/**
+	 * Filters the locale of the WordPress installation.
+	 *
+	 * @since 4.7.0
+	 *
+	 * @param string $locale The locale of the WordPress installation.
+	 * @return string The locale currently being switched to.
+	 */
+	public function filter_locale( $locale ) {
+		$switched_locale = end( $this->locales );
+
+		if ( $switched_locale ) {
+			return $switched_locale;
+		}
+
+		return $locale;
+	}
+
+	/**
+	 * Load translations for a given locale.
+	 *
+	 * When switching to a locale, translations for this locale must be loaded from scratch.
+	 *
+	 * @since 4.7.0
+	 *
+	 * @global Mo[] $l10n An array of all currently loaded text domains.
+	 *
+	 * @param string $locale The locale to load translations for.
+	 */
+	private function load_translations( $locale ) {
+		global $l10n;
+
+		$domains = $l10n ? array_keys( $l10n ) : array();
+
+		load_default_textdomain( $locale );
+
+		foreach ( $domains as $domain ) {
+			if ( 'default' === $domain ) {
+				continue;
+			}
+
+			unload_textdomain( $domain );
+			get_translations_for_domain( $domain );
+		}
+	}
+
+	/**
+	 * Changes the site's locale to the given one.
+	 *
+	 * Loads the translations, changes the global `$wp_locale` object and updates
+	 * all post type labels.
+	 *
+	 * @since 4.7.0
+	 *
+	 * @global WP_Locale $wp_locale The WordPress date and time locale object.
+	 *
+	 * @param string $locale The locale to change to.
+	 */
+	private function change_locale( $locale ) {
+		// Reset translation availability information.
+		_get_path_to_translation( null, true );
+
+		$this->load_translations( $locale );
+
+		$GLOBALS['wp_locale'] = new WP_Locale();
+
+		/**
+		 * Fires when the locale is switched to or restored.
+		 *
+		 * @since 4.7.0
+		 *
+		 * @param string $locale The new locale.
+		 */
+		do_action( 'change_locale', $locale );
+	}
+}