Is it possible to pay with euros in Eastern Europe? [duplicate]

I'm from Spain and I will be backpacking around Eastern Europe next month. I will visit some countries where Euro is the official currency (like Slovakia) and I'll also visit some other countries like Hungary or Serbia where they have a different currency.

I thought that it might be possible to pay using euros in all those countries because they receive a lot of tourists from Western Europe and they share borders with some euro-countries.

I think that I will be visiting only major cities.

So, my question is: Is it possible to pay using euros in those countries?

In case the answer is no or it is not recommended to do so (for instance, because is much more expensive to do so than to pay using the local currency), what would you recommend me to do?

P.S. I found this question which was helpful to me but I don't know if the situation has changed from the time it was asked or if the situation is different in some other countries.

EDIT: The countries that I'm planning to visit and don't have Euro as their currency are Hungary, Serbia, Croatia and possibly Romania. The situation in Hungary is now clear for me thanks to this question. But I would be still interested to know if the same applies in the other countries that I've mentioned.