Not every country in the EU is in the Eurozone. Given that the Single Market prevents EU countries from slapping each other with tariffs (Trump style), what prevents the countries in the EU but not in the Eurozone (e.g. Poland) from trying to devalue their currency and thus gain an (even short lived) economic advantage, especially over their Eurozone neighbors?
(On a broader international level, the IMF charter prohibits this, which doesn't mean it's not happening to some extent, worldwide.)