They think in whatever language they want--it probably isn't either "meow" (since that's something humans have applied to cats' sounds) or in English, it's probably more like "hungry, look for food" except not in words like that. Just like when you know you're hungry, usually your brain doesn't go "hey, you're hungry, go get something to eat"--it's just a feeling and they respond to it. However, when I make meow (or mew or some other similar cat sound), my cats stop and look at me and make other meow sounds and I try to mimic them--this can go on for quite a while. My male cat, siamese, will try to make the same sounds and in the same melody as I make (as long as it's close to the "m" sound since I think that's just how their vocal cords and mouths are formed)--I'll go "ma-meeee" and he'll try to say that. Or I'll go "meeeee-ow" and he'll try to do the same. He has a limited attention span so we don't get very far but he thinks its funny. My female will stop in her tracks if I mimic a sound she's making, and she'll sit and meow at me and wait for me to meow back at her and this can go on for a while--although she makes no attempt to mimic anything I try to get her to do, she just seems to be fascinated that I can make cat-like meow sounds. But, if I howl like a dog, both of them freeze and start to get scared. I must be good at this!