How do I remove unused language extension pages from my site?

[fancydressqueen]

Hi all,

I just checked my Google webmaster tools and it tells me that I have duplicate title tags, though they are in different languages, which I no longer have installed.

 

Here is an example:

 

http://www.otleyrunfancydress.co.uk/lang-de/330-black-halloween-fancy-dress-witch-costume-1.html

 

I only have English installed as a lanuage, so the German language page is not accessible to users. Can I delete this page or do I just add an addition to my robots txt file to prevent them being crawled?

 

Many thanks.