[@Oblik]
Ah, for the sake of accuracy:

The Juran know the Rurnian's language because they've been spying on them for ages, and have considerable experience with learning alien languages. Meanwhile, the Rurnians have never ventured out of their space (as you said), and thus would have no idea who the Jurans are... The Juran sent a message to them using Rurnian language, and so they wouldn't have even the slightest sample of the Juran language.

Just something to consider- you can't know what you've never seen or heard.