Once you enable Developer Mode under the settings tab, you will be able to connect to the following testnests: Goerli, Kotti, Kovan, Sokol, Rinkeby and Ropsten. Along with the the RSK and TomoChain testnets also.
Developer Mode Interface
Don't worry if you have no wish to interact with testnets, simply keep developer mode disabled and all of these networks will be hidden from view. Much less confusing if you want your Saturn Wallet's UI to remain focused on being used to trade tokens or interact with dapps across Ethereum, Ethereum Classic, TomoChain, xDai Chain, POA Network or RSK.