To be clear, Hutong Games doesn't collect any taxes. The Asset Store and Fastspring stores handle that for all sellers. They determine the sales tax based on the region, regulations etc. and submit any taxes collected to the appropriate authorities. Technically they are resellers - they buy from us and sell through their storefronts, handling the transactions (different payment methods, exchange rates, taxes etc.).
I have no idea what we would have to do if doing it all ourselves - I researched it for a bit but got nowhere. Some people sell direct through PayPal and don't collect taxes, but they might get stuck with a huge tax bill down the line. Depends who you talk to. Anyway, it seems like most legit companies add local taxes, so we use a storefront that handles that for us.
Hope that makes sense. I can't answer specific questions about taxes in one country or another because it's all a black box to us! We pay the storefront so we don't have to figure it all out!