There's a scene where the Meereenese rider is challenging Daenerys' champion. He's shouting and Nathalie Emmanuel [Missandei] is translating – but she's not translating what he's saying. He's actually saying a Low Valyrian translation of the French guy's insults in 'Monte Python and the Holy Grail.' That was [series creator] Dan Weiss's idea and it was so hilarious that I had to do it.
http://www.makinggameofthrones.com/production-diary/2014/5/8/interview-with-linguist-david-peterson