I had to fiddle about with this for a while using Google Translate to get what I needed as the blog instructions linked to the German site!
You have to get the API data from Spreadshirt to start with. I'm in the UK but the link in the installation notes (on the blog) is in German. Here is the link for the UK:-
Once you have the API, you can edit the shortcode on your site:-
[spreadplugin shop_id="[your shop ID]" shop_limit="20" shop_locale="en_GB" shop_source="co.uk" shop_category="" shop_social="1" shop_api="[get your API from the above link]" shop_secret="[get your shop secret from the link above]"]
Now, I changed the shop_locale to "en_GB" so it was in English and UK pounds.
I then changed the "shop_source" to "co.uk" as my shop is on the UK Spreadshirt site.
Hope this helps you.