Strange. This - or something close to this - comes up every once in a while, so I thought googling for an answer would be easy. Seems not so. There has to be DJP answer somewhere, but right out of my head the safest choice should be
Hash yer shili lekhes dothraki? or
Hash shafka shili lehkes dothraki?
latter being more formal way. That isn't exactly "Do you speak dothraki?" it's more "Do you know Dothraki?" More exact literal translation is more iffy, IMO, but goes:
Hash yer asti lekhes dothraki? or
Hash shafka asti lehkes dothraki?