You can buy an extension for PrestaShop or you can do it yourself, fix just two lines of code.
So, open the file /classes/tools.php and find this (twice):
Currency::getCurrencyInstance((int)$cookie->id_currency);
And replace it to
Currency::getCurrencyInstance(N);
where N is an id of currency (1, 2, 3 etc.).
That's all. Don't forget to save the file. Tested with PrestaShop 1.6.