The negative one? Perhaps you should leave the 230 Vac to someone else?
FR4 usually has a rating of 800 V / mil but this degrades to anywhere between 300 V / mil and 150 V / mil over time depending on the environment. For a double sided board the tracks on opposite sides of the board are probably the least of your worries.
But since you are interested in the requirements specified by the relevant safety standard it would seem you are interested in marketing this product. In that case, depending on the application, you are likely to have to submit your board for testing to see if it complies with the relevant standards in each country you plan to market in. This can get very expensive very quickly.