Открываем /classes/tools.php и находим строку

для 1.4

$currency = Currency::getCurrencyInstance((int)
(Configuration::get('PS_CURRENCY_DEFAULT')));

вместо неё вставляем

$currency = Currency::getCurrencyInstance((int)('1'));

для 1.5 

$currency = Currency::getCurrencyInstance((int)$cookie->id_currency);

вместо неё вставляем

$currency = Currency::getCurrencyInstance((int)('1'));

номер в конце - это ID той валюты, которую вы хотите сделать главной на сайте независимо от того какая главная валюта в админке.

Способ для 1.6

Создаём файл /override/classes/tools.php с содержанием

<?php
class Tools extends ToolsCore
{
public static function setCurrency($cookie)
{
if (Tools::isSubmit('SubmitCurrency') && ($id_currency = Tools::getValue('id_currency'))) {
/** @var Currency $currency */
$currency = Currency::getCurrencyInstance((int)$id_currency);
if (is_object($currency) && $currency->id && !$currency->deleted && $currency->isAssociatedToShop()) {
$cookie->id_currency = (int)$currency->id;
}
}

$currency = null;
if ((int)$cookie->id_currency) {
$currency = Currency::getCurrencyInstance((int)$cookie->id_currency);
}
if (!Validate::isLoadedObject($currency) || (bool)$currency->deleted || !(bool)$currency->active) {

$currency = Currency::getCurrencyInstance(1);
// $currency = Currency::getCurrencyInstance(Configuration::get('PS_CURRENCY_DEFAULT'));
}

$cookie->id_currency = (int)$currency->id;
if ($currency->isAssociatedToShop()) {
return $currency;
} else {
// get currency from context
$currency = Shop::getEntityIds('currency', Context::getContext()->shop->id, true, true);
if (isset($currency[0]) && $currency[0]['id_currency']) {
$cookie->id_currency = $currency[0]['id_currency'];
return Currency::getCurrencyInstance((int)$cookie->id_currency);
}
}

return $currency;
}
}

В строке $currency = Currency::getCurrencyInstance(1); вместо 1 вставить ID нужной валюты