There was an unusual situation where the wrong language appeared on the default versions of English buttons e.g. ..._checkout.png.
The English specific button was correct e.g. ..._en.checkout.png.
This problem happened when a user had created themes using a default language other than English, and then changed back to using English as the default.
In this situation the default versions of the images were not re-generated and stayed in the previous language.
This bug is now fixed (2013-01-04). if you are experiencing this problem, re-create the design to get a new set of images.
Alternatively, copy the images named *_en.* to replace the default versions without the "en." in their name.
You can also force the use of English specific images by adding lang="en" to an html tag that surrounds the Ecwid product browser widget. E.g.
... ecwid widget code ...