Google translates the entire thing as, "Do not disturb the sleeping cat, please. I have other fish to fry."
If you remove the first sentence, Google then translates the second sentence correctly. I could understand it 'correcting' the word 'cat' to 'fish' if it was using some kind of recognition of common expression algorithm, but not the behaviour as seen.
Update: If you have anything as a first sentence, it gives 'fish' in the second, instead of 'cats'. Then after more messing around, it always gives 'fish', no matter how you arrange it.