If you've read Metzger's Early Versions, then you've heard of the Sogdian version of the NT. And likely that's all you've heard.

If you want to know more about the language called Sogdian, now you can! Check out the Sogdian Primer. The intro notes that most Christian texts found in the Sogdian language are translations from Syriac.

More intros to Iranian languages are available at http://www.fas.harvard.edu/~iranian/.