VT800: Undefined array key "languageStrings" in /var/www/html/vtigercrm/vtlib/Vtiger/Deprecated.php on line 625

This warning happens a fair bit...

The Vtiger_Langauge_Handler::getModuleStringsFromFile() might return an empty array, hence the Warning.

return $strings['languageStrings'] ?? [''|null|array()]; would probably work - but not sure what the return should actually be if there is no array key.