Trilarion: I didn't think about doing that manually. An automated script would probably scale with the maintenance resources needed quite favorably so that it doesn't matter for how many items you need to do it. Anything less than automated would indeed be impractical. But if they do, they could even automatically display the equivalent amount in your local currency next to the USD price (the price that they will charge).
I don't want to say GOG should not support more regional currencies.
Just look at amazon how often they change prices for the millions of the products that they offer. Surely, they are not doing that manually. There are algorithms at work probably with some feedback loop including sales information and the effort to let them run is almost zero compared to the total effort of amazon.
I use
https://camelcamelcamel.com/ to check if a product I want to buy is currently high priced or low priced.
There are quite a few things that I suspect GOG does manually. I wouldn't be surprised if this was one of them. As for displaying a real-time local equivalent, it would still have to be implemented and ensured that it wouldn't break the site in some way (as all changes/introductions GOG have a tendency of doing so), effort that it could instead be used in supporting more currencies.
Amazon is a much, much bigger organization with more than 300000 employees, compared to GOG's meager 200. They can't be directly compared. Also, Amazon has a lot of localized webstores, something that can't be said for GOG.