Common mode Choke for AC voltage measurement
Not a good idea. It is not designed to carry much magnetic flux in the core at 50 Hz nor have the isolation between windings that you need.

Think about how the common mode choke is used. With live and neutral running through it in opposite phase, there would normally be very little resultant magnetic flux at 50Hz and all other frequencies. The core is only designed for the small induced RF currents which come in common mode.

The easiest solution is a mains to low voltage transformer, with safety agency approval (kilovolts of dielectric strength between windings). Other solutions are also possible eg voltage divider into an ADC on the live side, and pass the digital signal through an opto coupler to the rest of the circuit (low side micro). But then you have to make a small power supply for the ADC.