You haven't made it clear whether this is an eCommerce site or not - I'm assuming it is because you mention multi-currency.
If the site is primarily eCommerce then you might be better served by a dedicated eCommerce platform. I'm personally a fan of Opencart, which supports multiple currencies out of the box, and multiple languages via free/purchased extensions. Like Wordpress, it's PHP based, and you can implement mods via extensions (comparable to Wordpress plugins) or by developing your own mods. Another comparable platform worth looking at (although I haven't used it myself) is PrestaShop.
The quality of Wordpress eCommerce plugins varies wildly, I think, not sure what your client is running at the moment. From personal experience, some are much better and easier to customise than others (WooCommerce is much better than WP eCommerce, for example), but I've yet to see one that is anything like as comprehensive as a dedicated platform.
Hope that helps...