HBO

She speaks 19 languages, and why would she need to unless she had to speak to people all over the world?! Her emotions are also difficult to read, just like the Faceless Men, and she knows quite a bit of inside info for someone who was ~just a slave~ for so long before joining Dany's circle. Not to mention her saying "Valar Morghulis" — which, yes, could be a common phrase — but is specifically the one given to Arya by Jaqen. There's SO MUCH MORE to this theory, so jump down the rabbit hole.

—noodleslinger