Depends where you live. As Voices said, the Japanese word for the sound a cat makes is 'Nyan'. The English word is 'meow'. It just so happens that the English word sounds like what they actually say - the word isn't meant to be what cats say, it's the name for the sound, if that makes sense. Like, the French word for 'woof' is 'trame'. :3
So, for me, 'meow' all the way.
And yeah, if English-speakers say 'nyan', it's probably due to this video