I had the same problem with the danish translation. I also had the problem with the login form on the checkout page which contains this string
"If you have shopped with us before, please enter your details in the boxes below. If you are a new customer, please proceed to the Billing & Shipping section.".
I found that the problem occured because the strings in the .po file had been changed since the latest danish translation. So "Postcode / Zip" is now "Postcode / ZIP". So when i replaced the source strings in my danish translation to the correct strings, it worked again.
