Cześć Krzysztof,
Thanks for reaching out again with your questions. Let me try to help you out with that.
You have more options to order optional products, depends on how you want to execute it. If you want to have it directly on the event page with the ticket, then the easiest option would be setting up different tickets (Ticket only, Ticket + book, Ticket + gadget, Ticket + book + gadget etc.). If you have a lot of options, then this can be cumbersome.
You can try to sell tickets not from the Event page, but rather from the WooCommerce product page. Here’s a guide on how you could do that:
Sell Tickets From the WooCommerce Products Page
Once you have this in place, then a plugin like WooCommerce Product Add-ons or the YITH WooCommerce Product Add-ons could help you achieve what you want with the books and gadgets. These are not free plugins, so would require some investment from your side.
Add to cart:
You should be able to translate this with the .po file. This knowledgebase article should help you in this:
How Do I Update the Calendar’s Language Translations?
Let me know if you have any other questions.
Cheers,
Andras