Source

File: wp-includes/class-wp-locale-switcher.php

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;
	}