For the accessible foreign stock exchanges, it is possible to buy securities denominated in a currency other than the Euro (for example an american share denominated in US-Dollars). However, if you do not have a current account in the currency concerned, a foreign exchange transaction will be carried out to acquire your securities. Depending on the evolution of the exchange rate of the currency against the Euro, you have a foreign exchange risk if you want to convert your capital back into Euros. As exchange rates can  fluctuate quickly, you have to keep an eye on this risk or limit yourself to purchases in the currency of the associated current account. It is also possible to open an account in the currency in question, if you intend to make regular transactions in that currency.