In English, the word 'wag' is not used for the movement of a cat's tail. The action of the movement and its intent are different enough that it feels very wrong to use for cats.
However, the answer for the word for the movement of a cat's tail is not so obvious. The only movement of a cat's tail that is close to that of a dog's is used by the cat, not when it is happy or conciliatory (as it is for a dog) but when it is in attack or stealth mode. The word to describe this would be
to flick its tail back and forth
'flicking' is a general movement, not closely connected with a cat (as opposed to a dog where the word 'wagging' is almost a defining characteristic).