If your site went down right after the last Polylang update with this error:

PHP Fatal error: Uncaught TypeError: Cannot assign null to property WP_Syntex\Polylang\Switcher\Element\Abstract_Element::$locale of type string in .../plugins/polylang/src/Switcher/Element/Abstract_Element.php:158

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…then it is not your theme, and it is not a broken update in the usual sense. One language row on your site is missing its locale , and 3.9.x turned that silent bad row into a fatal.

I traced it through the 3.9.1 source. Three lines matter.

1. The typed property that fails

src/Switcher/Element/Abstract_Element.php

public string $locale ; // line 47

$this -> locale = $language -> get_locale ( 'display' ); // line 158

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get_locale( 'display' ) returns $this->w3c ( src/language.php:608 ). If w3c is null , assigning it to a string property is a TypeError, and TypeErrors are fatal.

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