Description of problem:
PressGang does not yet handle translator credits

Version-Release number of selected component (if applicable):
Build 201309081012

How reproducible:
100%

Steps to Reproduce:
1. Try to assign translator credit


Actual results:
Do not find a way

Expected results:
Be able to specify the translator of a topic, just like specifying the author. translator should also specify to which language the credit belongs. Translator credits should be aggregated into the Author_Group.

Additional info:
It would be good to automatically get this information from Zanata, Transifex, or by parsing PO files. Translations have their own (derivative) copyright, so we would need to record copyright holder and licence terms as well (and ultimately automate safeguards against combining incompatibly licenced material)