When digging into T320821 we realised that over 500k of records have the invalid 'en' locale set. These records will throw an exception if they hit this code:
public static function negotiate(string $preferred): Locale {
// Create a locale for the requested language
if (!preg_match(';^[a-z][a-z]_[A-Z][A-Z]$;', $preferred)) {
throw new \RuntimeException("Cannot instantiate malformed locale: $preferred");
}