The Common Tongue is spoken in Westeros, but that's not the case across the Narrow Sea. Missandei's translations for Daenerys and her entourage are the work of David Peterson, the linguist who created Dothraki, High Valyrian and multiple dialects of Low Valyrian. By his own estimate, Peterson has crafted over 5,000 words.  Peterson shares the unique challenges of creating languages for the show, writing dialogue for Dany and what 'Monty Python' has to do with Meereen.