I have not been to Sweden in a few years, but there are generally two problems with American credit cards in western Europe, problems that have not yet migrated to eastern Europe.
One is the need for a PIN to use a credit card. Most US based cards do not routinely assign PIN's to credit cards, but will if you request one. If traveling in Europe, you should definite do this to be on the safe side.
The second is the embedded chip that credit cards issued in EU countries have. Machines that take credit cards often will not work unless you have a card with a chip. Since I am living on this side of the pond, I have two European-issued Visa cards, one from a country that embeds the chip and one from a non-EU country that does not have a chip. If my US-based Cap One card does not work, I pull out the Visa with the embedded chip, and it does.