It worked the above issue thank you, but still having issues:
The second issue seems to be that the Date Picker uses a different date format than what is recorded in Gibbon for the pl_PL locale. What is the standard form of writing a date in Poland? If it's dd.mm.yyyy, you could possibly correct this issue by running the following SQL on your database:
UPDATE `gibboni18n` SET `dateFormat` = 'dd.mm.yyyy', `dateFormatRegex` = "/^(0[1-9]|[0-9]|3)[- /.](0[1-9]|1)[- /.](19|20)\d\d$/i", `dateFormatPHP` = 'd.m.Y' WHERE `code` = 'pl_PL';
Our format is 11/06/2019 for example
How can i fix this, i still cannot continue with invoices because of the date issue.