Transcription use case: translations of transcriptions

[tristanr-cogapp]

For the Qatar Digital Library (www.qdl.qa) we have scanned documents in both English and Arabic. Although we do not currently have them, we would like to create translations of transcribed documents. If these were available, it would be good to be able to display a translated document.

Note that this would get even more complicated if combined with the request for overlaid transcriptions in #17 due to the left-to-right and right-to-left reading directions of each language.