As long as you use any cache plugins, the plugins will cache everything, including the currency cookies and the currency switch will show your chosen currency only after cache is deleted from plugin.
Our hosting company installed varnishcache and memcache server directly (not through a plugin). And this caching system doesn’t affect the multi-currency widget. You can test here http://wprentals.org/
The multi currency system DOES NOT work in infobox for technical reasons.
How to add the widget:
Go to Admin – Widgets and select the Multi-Currency Widget and on what sidebar you wish to display it. Can be added to top bar widget (DO NOT FILL IN TITLE to fit the widget in the top bar) or on any other sidebar
Currencies are set in Theme Options – Price $ Currency
For Currency Value use point not comma. –> 1.3339
Make sure the CODE are typed according to international standards (which means 3 letters – capslock) – you can find the complete list here http://www.xe.com/iso4217.php.
The dropdown works for front end currency conversion: