The templates inside x5 can be freely used, but websites should be made with x5.

If you buy templates there are more rules, but acknoladgement is not needed IMHO.

read also: the terms of use here in the helpdesk (on the right below) https://www.incomedia.eu/policy/terms-templates_en.html

Hi Steve!

As kindly mentioned by Andre, you can use as you please the templates in the template gallery of WebSite X5. 
If you wish to purchase Graphic Templates or Full templates from our Marketplace, you are entitled to create websites for your clients and you do not have to do anything else but please read this section carefully first https://market.websitex5.com/en/faq#templates

''The WebSite X5 templates that can be freely downloaded don't have any particular limits on use. You can use them for all the websites you want, including those you create for other people.

If you buy a Full Template, however, you can only use it for one website, regardless of whether this is intended for personal or commercial use (for example, for a client). If you wish to use the same Template for different projects, you must purchase a user licence for each one.''

I hope this helps but please feel free to write back if you have anymore questions. 
Thank you and have anice day!
Samantha