The supported way to translate the label is using the translation process (an example can be found here)
Maybe you can consider to create an unmanaged solution that contains only that entity, in order to export only the necessary labels.
There is also a tool recently released by Tanguy Touzard in his toolbox, I didn't use it yet, but you can find more information here:
http://mscrmtools.blogspot.com/2013/08/new-xrmtoolbox-plugin-easy-translator.html