Since the tax can vary on the customers location the tax is applied to the line item, not the product. But the product needs to know which kind of tax is applied to it for each country. For example in Germany there is a reduced tax for books and food etc. and a standard tax for pretty much everything else. It's quite similar in England, but they have even more tax rates. This means the tax depends on the country and the product type where the vendor is situated but also to the country where the customer is situated (or where the product is delivered to, I am not sure for 100% TODO). When the tax is applied (display in store front or QOI) it depends on the customers country and the product type, so the product needs to tell the tax rules which tax to apply. Anyway these rules need to be respected as they vary in each country. It should be possible to calculate automatically (store front) but also to let the user select the tax directly in the line item (backend for QOI).
Entering the correct tax rates for each country on each product is the responsibility of the user editing the products.