Fix the handling of invalid content language ids

Author: dtdesign

wcfsetup/install/files/lib/acp/form/UserAddForm.class.php

@@ -249,15 +249,15 @@ public function validate()
 
         // validate user language
         $language = LanguageFactory::getInstance()->getLanguage($this->languageID);
-        if ($language === null || !$language->languageID) {
+        if ($language === null) {
             // use default language
             $this->languageID = LanguageFactory::getInstance()->getDefaultLanguageID();
         }
 
         // validate visible languages
         foreach ($this->visibleLanguages as $key => $visibleLanguage) {
             $language = LanguageFactory::getInstance()->getLanguage($visibleLanguage);
-            if (!$language->languageID || !$language->hasContent) {
+            if ($language === null || !$language->hasContent) {
                 unset($this->visibleLanguages[$key]);
             }
         }